there are 4 department groups in an SV9100, 1 stopped working yesterday while nobody touched it. when you rang the department pilot, you got beep. So we re-created the department with same pilot in another department group, it fixed some issue, at least we could ring the pilot.

This morning they had another issue:
"Extension 363,364,365 when all 3 phones ring with the same call, if you answer the call on one of the extensions the call continues to ring on the other extensions , you cannot answer the other phones if you do you interrupt the call being answered.
Once the call is finished the other phones then stop ringing."
by the way, 363-365 are in the department.

It's weird. Any idea?

A few. Extensions cannot have ANY forwarding on for the dept group to work (DND etc). Are you on the latest software? Check your call coverage keys (if you are using them) to make sure they are unique (someone didn't copy function keys).