I am seeing the exact same issue, no problem on Prestashop, but thirty bees does not process the order.  
I see the post back but the Cart Does not get turned into a valid order 
105.186.16.177 - - [27/Feb/2018:13:02:47 +0200] "GET /modules/paynow/logo.png HTTP/1.1" 200 3326 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10133) AppleWebKit/604.5.6 (KHTML, like Gecko) Version/11.0.3 Safari/604.5.6"
41.21.175.145 - - [27/Feb/2018:13:06:12 +0200] "POST /modules/paynow/paynowcallback.php?key=xxxxxxxxxxx0f716d7fa8cd8b3064&idcart=23188&idmodule=185 HTTP/1.1" 302 4708 "-" "-"
105.186.16.177 - - [27/Feb/2018:13:06:13 +0200] "POST /modules/paynow/paynowcallback.php?key=xxxxxxxxxxx0f716d7fa8cd8b3064&idcart=23188&idmodule=185 HTTP/1.1" 302 630 "https://paynow.sagepay.co.za/Site/PayNowAccept.aspx" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10133) AppleWebKit/604.5.6 (KHTML, like Gecko) Version/11.0.3 Safari/604.5.6"
105.186.16.177 - - [27/Feb/2018:13:06:14 +0200] "GET /index.php?controller=order-confirmation?key=xxxxxxxxxxx0f716d7fa8cd8b3064&idcart=23188&idmodule=185 HTTP/1.1" 302 389 "https://paynow.sagepay.co.za/Site/PayNowAccept.aspx" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10133) AppleWebKit/604.5.6 (KHTML, like Gecko) Version/11.0.3 Safari/604.5.6"
105.186.16.177 - - [27/Feb/2018:13:06:14 +0200] "GET /order-history HTTP/1.1" 200 7936 "https://paynow.sagepay.co.za/Site/PayNowAccept.aspx" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10133) AppleWebKit/604.5.6 (KHTML, like Gecko) Version/11.0.3 Safari/604.5.6" 
I do also get a warning on the dispatcher...  
[Tue Feb 27 13:06:13.512467 2018] [fcgid:warn] [pid 160555] [client 41.21.175.145:55652] modfcgid: stderr: PHP Warning:  Invalid argument supplied for foreach() in /var/www/clients/client2/web5/web/classes/Dispatcher.php on line 513
[Tue Feb 27 13:06:13.959738 2018] [fcgid:warn] [pid 160555] [client 41.21.175.145:55652] modfcgid: stderr: PHP Warning:  Invalid argument supplied for foreach() in /var/www/clients/client2/web5/web/classes/Dispatcher.php on line 513 
This is what I get in the module debug log 
2018-02-27 13:06:12 : Verify data received
2018-02-27 13:06:12 : Host = gateway.sagepay.co.za
2018-02-27 13:06:12 : Params = RequestTrace=36973.31290079
2018-02-27 13:06:12 : curl_init
2018-02-27 13:06:13 : Response: 
2018-02-27 13:06:13 : Amount: Valid!
2018-02-27 13:06:13 :  --> valOne: [string] true
2018-02-27 13:06:13 :  --> valTwo: [string] true
2018-02-27 13:06:13 : TransactionAccepted: Valid!
2018-02-27 13:06:13 : Reference: Valid!
2018-02-27 13:06:13 : RequestTrace: Valid!
2018-02-27 13:06:13 : pnValidData: true
2018-02-27 13:06:13 : Check status and update order
2018-02-27 13:06:13 : - Complete
2018-02-27 13:06:13 : - Redirecting to order-confirmation
2018-02-27 13:06:14 : SagePayNow Validation initiated.
2018-02-27 13:06:14 : Get posted data
2018-02-27 13:06:14 : SagePayNow Data: Array
(
    [TransactionAccepted] => true
    [CardHolderIpAddr] => 198.6.6.1
    [RequestTrace] => 36973.31290079
    [Reference] => 23188_2018022746
    [Extra1] => 1337
    [Extra2] => 23188
    [Extra3] => xxxxxxxxxxx0f716d7fa8cd8b3064
    [Amount] => 3765.0000
    [Method] => 1
) 
2018-02-27 13:06:14 : Purchase:
Cart Object
(
    [idshopgroup] => 1
    [idshop] => 1
    [idaddressdelivery] => 1378
    [idaddressinvoice] => 1378
    [idcurrency] => 3
    [idcustomer] => 1337
    [idguest] => 576807
    [idlang] => 1
    [recyclable] => 0
    [gift] => 0
    [giftmessage] => 
    [mobiletheme] => 0
    [dateadd] => 2018-02-27 12:57:25
    [securekey] => xxxxxxxxxxx0f716d7fa8cd8b3064
    [idcarrier] => 28
    [dateupd] => 2018-02-27 13:02:33
    [checkedTos] => 
    [pictures] => 
    [textFields] => 
    [deliveryoption] => a:1:{i:1378;s:3:"28,";}
    [allowseperatedpackage] => 0
    [products:protected] => 
    [taxCalculationMethod:protected] => 0
    [webserviceParameters:protected] => Array
        (
            [fields] => Array
                (
                    [idaddressdelivery] => Array
                        (
                            [xlink_resource] => addresses
                        ) 
                [id_address_invoice] => Array
                    (
                        [xlink_resource] => addresses
                    )
                [id_currency] => Array
                    (
                        [xlink_resource] => currencies
                    )
                [id_customer] => Array
                    (
                        [xlink_resource] => customers
                    )
                [id_guest] => Array
                    (
                        [xlink_resource] => guests
                    )
                [id_lang] => Array
                    (
                        [xlink_resource] => languages
                    )
            )
        [associations] => Array
            (
                [cart_rows] => Array
                    (
                        [resource] => cart_row
                        [virtual_entity] => 1
                        [fields] => Array
                            (
                                [id_product] => Array
                                    (
                                        [required] => 1
                                        [xlink_resource] => products
                                    )
                                [id_product_attribute] => Array
                                    (
                                        [required] => 1
                                        [xlink_resource] => combinations
                                    )
                                [id_address_delivery] => Array
                                    (
                                        [required] => 1
                                        [xlink_resource] => addresses
                                    )
                                [quantity] => Array
                                    (
                                        [required] => 1
                                    )
                            )
                    )
            )
    )
[id] => 23188
[id_shop_list] => 
[get_shop_from_context:protected] => 1
[table:protected] => cart
[identifier:protected] => id_cart
[fieldsRequired:protected] => Array
    (
        [0] => id_currency
        [1] => id_lang
    )
[fieldsSize:protected] => Array
    (
        [secure_key] => 32
    )
[fieldsValidate:protected] => Array
    (
        [id_shop_group] => isUnsignedId
        [id_shop] => isUnsignedId
        [id_address_delivery] => isUnsignedId
        [id_address_invoice] => isUnsignedId
        [id_carrier] => isUnsignedId
        [id_currency] => isUnsignedId
        [id_customer] => isUnsignedId
        [id_guest] => isUnsignedId
        [id_lang] => isUnsignedId
        [recyclable] => isBool
        [gift] => isBool
        [gift_message] => isMessage
        [mobile_theme] => isBool
        [allow_seperated_package] => isBool
        [date_add] => isDate
        [date_upd] => isDate
    )
[fieldsRequiredLang:protected] => Array
    (
    )
[fieldsSizeLang:protected] => Array
    (
    )
[fieldsValidateLang:protected] => Array
    (
    )
[tables:protected] => Array
    (
    )
[image_dir:protected] => 
[image_format:protected] => jpg
[def:protected] => Array
    (
        [table] => cart
        [primary] => id_cart
        [fields] => Array
            (
                [id_shop_group] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_shop] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_address_delivery] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_address_invoice] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_carrier] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_currency] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                        [required] => 1
                    )
                [id_customer] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_guest] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                    )
                [id_lang] => Array
                    (
                        [type] => 1
                        [validate] => isUnsignedId
                        [required] => 1
                    )
                [recyclable] => Array
                    (
                        [type] => 2
                        [validate] => isBool
                    )
                [gift] => Array
                    (
                        [type] => 2
                        [validate] => isBool
                    )
                [gift_message] => Array
                    (
                        [type] => 3
                        [validate] => isMessage
                    )
                [mobile_theme] => Array
                    (
                        [type] => 2
                        [validate] => isBool
                    )
                [delivery_option] => Array
                    (
                        [type] => 3
                    )
                [secure_key] => Array
                    (
                        [type] => 3
                        [size] => 32
                    )
                [allow_seperated_package] => Array
                    (
                        [type] => 2
                        [validate] => isBool
                    )
                [date_add] => Array
                    (
                        [type] => 5
                        [validate] => isDate
                    )
                [date_upd] => Array
                    (
                        [type] => 5
                        [validate] => isDate
                    )
            )
        [classname] => Cart
    )
[update_fields:protected] => 
[force_id] => 
) 
2018-02-27 13:06:14 : Verify data received
2018-02-27 13:06:14 : Host = gateway.sagepay.co.za
2018-02-27 13:06:14 : Params = RequestTrace=36973.31290079
2018-02-27 13:06:14 : curl_init
2018-02-27 13:06:14 : Response: 
2018-02-27 13:06:14 : Amount: Valid!
2018-02-27 13:06:14 :  --> valOne: [string] true
2018-02-27 13:06:14 :  --> valTwo: [string] true
2018-02-27 13:06:14 : TransactionAccepted: Valid!
2018-02-27 13:06:14 : Reference: Valid!
2018-02-27 13:06:14 : RequestTrace: Valid!
2018-02-27 13:06:14 : pnValidData: true
2018-02-27 13:06:14 : Check status and update order
2018-02-27 13:06:14 : - Complete
2018-02-27 13:06:14 : - Redirecting to order-confirmationxxxxxxxxxxx