I need expert help fixing a problem with Virtuemart user registration

[steveprice]

I have a very frustrating intermittant problem that I can ot solve.  I need some help.

The problem works like this:

1. Non registered customer adds products to cart and proceeds to checkout by clicking the green "Checkout Now" button. (good)
2. User is sent to /index.php/video-on-demand-ppv/user/editaddresscheckoutBT where he can complete his details and click "register and Checkout" (excellent)

After the user has clicked "register and Checkout" it goes wrong. I either get:

3. the user is returned to the cart but gets the red error message "User name in use".

(A quick look in the admin end shows that the user was in fact registered correctly at this point - it is the error message that is wrong. Now, of course the user can't simply try with a different username because his email is already registered to the system from his first attemt.)

Or i get:
3. The user is registered and shown the message "Registration Complete" but is taken back to editaddresscheckoutBT to confirm his email when he should just go straight back to the cart to complete the order.


Or (rarely) I get:
4. The user is registered Ok and taken through the checkout correctly. 

I have only seen the problem  in Internet Explorer.  I have set up 2 test sites on different servers and get exactly the same problem on both.

I need someone to work out why this is happening and fix it please.