then you should thank APEX 5 for finding (and no longer tolerating) a logic bug in your application.
|APEX 5 - correcting your applications|
Track down that field and set the Source Used attribute to 'Always, replacing any existing value in session state'.
On a side note, setting the application's Compatability Mode to 4.2 will also fix the problem, but in this case certainly not the appropriate solution. But this means by setting this attribute to 5.0 on your existing applications can help find these problems in the first place.