If you attempt to revise a project from the Schedule tab, where the Gantt chart is visible, your session is expired and you receive an invalid session error. The issue was observed in Internet Explorer and Chrome, but not in Firefox.
An analysis from a Fiddler trace shows that when revising the project in Chrome, this POST to GanttDataUpload.jsp seems to kill the session. In Firefox, for whatever reason, this POST doesn’t occur, and the state transition is successful. To confirm that this is the scenario you are experiencing, use the following technote to run a Fiddler trace and check for the same GanttDataUpload.jsp call: IBM TRIRIGA using Fiddler for tracing web browser traffic.
As a temporary fix, use Firefox. When the record is in a read-only state, no Save action should be called on the Gantt. Moving forward, we resolved the session-kill issue when the user performs a Revise action on a project in the Schedule tab.