Intermittent issue with shipping

[movieseals]

Hi again!

A new issue has popped up.

Customers get the following message sometimes when attempting to check out: 

THERE IS 1 ERROR

1. THE PRODUCT SELECTION CANNOT BE DELIVERED BY THE AVAILABLE CARRIER(S): IT IS TOO HEAVY. PLEASE AMEND YOUR CART TO LOWER ITS WEIGHT.

I have searched the forum and some other users mentioned that issue but their fix do not seem to work for me at the moment.

The weight is way below the limits in all cases and the settings are the same they have always been (and working without issue before).

Even worst, the issue is intermittent - sometimes it happens, sometimes it doesn't and I have not been able to figure out the pattern.

Anybody else has encountered this and managed to fix this issue?  Losing lots of sales because most people just abandon their cart instead of contacting us.

[movieseals]

OK.  It appears that the issue is related to guest or visitors logins - if a user is logged in with his account as a customer, there is no error message.  However, if he first add items to his cart as a guest or visitor, until he actually logs in, the following error message appears: THE PRODUCT SELECTION CANNOT BE DELIVERED BY THE AVAILABLE CARRIER(S): IT IS TOO HEAVY. PLEASE AMEND YOUR CART TO LOWER ITS WEIGHT.

No matter what shipping I set up, including pickup in store which has a limit of 1000 kg, this message appears...

It prevents orders from being completed and it freaks customers out.  Help!

[Rhapsody]

What theme are you using?  I had a similar problem with a modified community default theme and found the solution in this post.

[toplakd]

Are your carriers enabled for all groups or just for the customer group?