123V is not going to hurt anything that was meant to run at line voltage. If that UPS is glitching and cutting out a few percent over voltage, you’re better off without it. How often does your power cut out comparred to how often this glitch happens.
It’s my understanding that uniteruptable power supplies are supposed to protect you from spikes, voltage drops, and power loss. Yours should not be acting this way.
I agree with z3n’s post. It’s better to just replace the UPS.