When Gabriele and I were trying to lock the IFO this morning, the ISC_LOCK guardian dead at around Sep 08 19:16:10 UTC. The symptom was exactly the same as described in LHO:43322 except for that this time it was for ISC_LOCK. It seemed to be related to some virtual circuit disconnection. We don't know what caused the issue exactly.

For now we were able to fix it by running:

guardctrl restart ISC_LOCK