OS Reinstall worked ... UPS issue?

I have an Eaton UPS that's directly connected to my main NAS via USB and to my other NAS via SNMP, so my set-up is similar, but not exactly the same.  The big difference is that the secondary NAS doesn't need to talk to the primary, it talks directly to the UPS network card.  But if that connection is lost, the secondary NAS simply complains of the lost connection and won't shut down because it doesn't get the command -- it has no impact on the ability to power it back up.

I suspect you have something else going on that only became visible only because of the cycled power.  Two things that can cause an almost immediate shut-down are a voltage out of range and a fan not detected as turning.  Given it seems to have corrected itself after a couple tries, my first suspect is a failing fan.  But the fact that it seemed to work without drives and not with points more toward a power issue -- maybe the power brick and maybe a secondary voltage created form it.   Keep an eye on the log for any indication of a low voltage or fan issue.