Just replaced my C8 with a C8-Pro and started noticing Z-Wave issues a few hours later. Luckily I came across this thread and switched to the included USB brick which resolved my issues.
I was using a Mini USB UPS from Amazon (mine was an older version).
I just ordered the "Procet Gigabit PT-ATC-5V" since Amazon won't have the PTC-AT till mid January.
Hoping it will play nice with my UniFi USW 24 PoE. I read through the entire thread and saw that Ubiquiti can cause issues so I hope that is not the case.