suddenly 'whoops your session has expired' with Secure Trading payments

Since last night, we have had issues with a customers payment system. They use secure trading, and on checkout are whisked off to secure trading to pay, enter their details and then are sent back to the zencart.

That still happens, but when they are taken back to zencart it says 'whoops your session has expired'.

No settings that we know of have been changed, and we have the recommended session values in configuration.

Anyone have any ideas?

Also is there a way to check that the zenid has been passed to the secure trading form?

thanks

Infact the message on return to zencart is:

hoops! Sorry, but you are not allowed to perform the action requested.

The url in the browser when it comes back to zencart is:

/index.php?main_page=time_out&zenid=cbef15e5a88c9bb126284aec64173aaa

so the zenid is there. But why is it timing out? and not going to checkout success?

Ive checked the callback file which is correct: /index.php?main_page=checkout_process

What reason would it go to timeout?

ok fixed. Turns out secure trading had messed up the files located on their ftp server......!