(this question might be better suited for the User Experience site)
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.