Whoops! Your session has expired.

I keep getting this error message after credit cards are processed using authorize.net(SIM). (AIM is not an option for my cart)

Zen Cart 1.5.1 (new install)
PHP Version: 5.4.16
MySQL 5.6.10

When checking out, customers are redirected to my payment services provider (eProcessingNetwork) to enter their card information. Once information is entered and submitted, the chard is charged and the customer sees an unformated version of my cart page with the error message:

"Whoops! Your session has expired. If you were placing an order, please login and your shopping cart will be restored. You may then go back to the checkout and complete your final purchases."

The URL still shows the eProssessingNetwork address.

I have read many posts about this error but most only see it occasionally. This happens with every credit card transaction and none of the solutions have helped.

How did you resolve this? I am getting same error every time I use authorize.net SIM

If you're using an outdated template, that would cause the problem.
If you're using addons for an older version of ZC, that could cause it.

1. You didn't say what version of ZC you're using,
2. You didn't list your addons.
3. Can you also tell us whether you're using Authnet SIM as-is, or whether you've modified it to work with another gateway somehow.
   Related FAQs: http://www.zen-cart.com/content.php?303-how-to-set-up-the-authorizenet-sim-payment-module and http://www.zen-cart.com/content.php?302-authorizenet-sim-an-error-occurred-while-trying-to-report-this-transaction-to-the-merchant-an-e-mail-has-been-sent-to-the-merchant-informing-them-of-the-error-the-following-is-the-result-of-the-attempt-to-charge-your-credit-card

Here is the info:

1. You didn't say what version of ZC you're using,
   I am using 1.5.3

2. You didn't mention your PHP version,
   5.4.31

3. You didn't list your addons.
   New install. NO addons

4. Can you also tell us whether you're using Authnet SIM as-is
   absolutely no changes to the Authorize.net SIM that came included with ZenCart

Unfortunately I can't replicate the problem using a clean install of ZC v1.5.3 and PHP 5.4 or 5.5.
Usually that means it's something odd with the host.

But if it's possibly related to settings, these FAQ articles may offer some ideas for you:
http://www.zen-cart.com/content.php?302-authorizenet-sim-an-error-occurred
http://www.zen-cart.com/content.php?303-how-to-set-up-the-authorizenet-sim-payment-module

Is this still happening? Are you hosted with Dreamhost?

Check this PHP configuration setting:

1. Admin->Tools->Server Info ... Press CTRL+F to "search the page" and search for session.hash_bits_per_character
2. If session.hash_bits_per_character is set to 6, then you'll need to apply this fix: http://www.zen-cart.com/showthread.php?215694&p=1270902#post1270902