It's not the fan header or electrical capability of the controller itself, but the current monitoring chip (mine errors out while the Vardars "boot up" even on channels that do not have load, only pump PWM signal). I've confirmed this with Aqua Computer support and they've seen issues with Vardars that behave this way.
From an EE perspective I do not see why having an UPS (unless its a Double Conversion Online type - which aren't for home use) has anything to do with this topic. A typical Line Interactive UPS has 1-2 steps buck-boost and doesn't do much when power fluctuates (it also goes through 0V and relies on the PSU to hold up power until it switches to battery). I think you are stuck on electrical problems generated by me disconnecting and reconnecting the fans
The reason I opened this topic was to know if the EVOs behave the same way in case I decide to replace my fans.