I'm having the same issues, GH will work manually for about 30 seconds, then all devices show offline again. I don't this the "action" mention above is working as stated.
I updated my ISY from 5.3.0 to 5.34. Now it only boots in safe mode. I don't use Insteon (All Z-wave), so no PLM. I have a good backup, but that doesn't help. I even did Factory reset, and it remain 5.3.4 and looks for PLM.