Validation error when creating or editing nodes

I have been getting reports of the following error occurring regularly when users are trying to create or edit any nodes (not specific to any content types):

"Validation error, please try again. If this error persists, please contact the site administrator."

After researching this error on drupal.org and Google, it seems the issue might be related to caching and/or sessions. But I haven't been able to replicate the error myself (I primarily use Firefox 3 and tend to clear the cache every time I close the browser). Users have reported needing to logout then back in again to "clear" the error. I tried checking the "recent logs" but it looks like these types of errors aren't logged(?) and I'm not familiar with other ways to troubleshoot the problem or where I should be narrowing my search (core or modules or both?).

Here are some specs for the site:

Drupal 5.12, Webauth (specific to our university), CCK, Content Access, Devel, Event, FCKeditor, Javascript
Tools, IMCE, Token, Webform, Update Status, Views, CAPTCHA

Roles: Users experiencing problems are logged in and have 2 roles - admin (full access) and webauth user - very basic "access content" permission)

Browsers: Primarily IE7 - some Firefox 3

Caching: All site caching turned off in admin settings

Two fixes that I have attempted so far:

1. In the settings file, I edited line 130: ini_set('session.cache_expire', 0);
2. In the .htaccess file, increased the php maximum execution time to 300

Any help would be greatly appreciated - Thanks!