*(this question might be better suited for the [User Experience site](http://ux.stackexchange.com/))*

**Change the error message to "Your session has expired because you were inactive for more than XX minutes."** Problem solved.

Let the user know how long they have exactly, because if they have to submit a complex form they need to be aware that they will need to collect whatever information they need and prepare whatever text they must input within a set time. Even better, monitor on which forms sessions tend to reset and use that information to figure out if you must support longer-lasting sessions.