I had a problem with the SSH inodes sensor coming back with no information after I rebooted 4 servers. The sensor was greyed out most of the time, but would also bounce into an error state some.
I tracked it down to not having one of the NFS shares mounted. It was a little used share which didn't automount on restart. When I mounted it, the sensor immediately recovered. I thought this might be useful information to have. Also, maybe this scenario could show a more meaningful error for debugging in the future.