Drupal 7.10 : views filters error
This is a very common and very difficult to debug problem. It happens when there's an inconsistency between the cache version and the submitted version of a form; whatever the reason.

You're best bet would be to upgrade Views and Ctools (and perhaps BEF too) to the latest dev versions.

Also, one important thing to consider is wether or not you have custom code altering the form.