If a customer wants to order (without login or guest checkout), no error message will appear when clicking on the "buy" button. For example, it lacks the hint "You need to log in, create an account" or something like that.
The curious thing is, with the payment options bank transfer and Stripe, the page is simply reloaded without an error message, which gives the customer the hint, you must log in or something.
When paying with Paypal you will be forwarded directly to the Paypal payment. The customer pays the order. Thereby also no order is generated in the shop.
tb 1.0.3 standard theme with AEUC module and OPC. Paypal is only set as default. Express checkout and Paypal login are not activated.