What is the control?
If it is a Fanuc, and the 2091 is on the operator message screen as opposed to something like a DS2091 which would be an alarm. This is caused by logic in the ladder. Look at the PMC diagnostics (status screen) under the "A" address'. When you find an "A" that is true (1) look in the ladder for that address (ex A2.7 and assuming straight forward logic). That should get you started in figuring out what is actually causing the issue.