Customer account creation and checkout problems

Hello,

I'm having a couple of major problems with Zen Cart 1.3.7:

It's a relatively fresh install (not upgraded) and most mods are stylesheet based or template override. I also have the Big Royal Mail 2.11 shipping module installed (though have now uninstalled this in case it was causing conflict - it doesn't appear to have been as the problem is still there)

The problem I have is with customer account creation and the final step of checkout - both actions cause a long delay and result in the following error message in the browser window (IE7)

Warning: Unknown(): A session is active. You cannot change the session module's ini settings at this time. in Unknown on line 0

However, both the customer account and order do appear in ADMIN.

Everything did appear to be working fine after the initial install but this problem (coincidenatally or not) started to manifest itself after installing Big Royal Mail 2.11. I say started because the first problem I noticed was that when a customer placed an order, ADMIN would get a copy of the order email but the customer wouldn't.

The website is not live all testing has been performed with 'dummy' user accounts.

Many thanks in advance,

Jonathan