UPS loses power every Sunday

[Silasch]

Hello dear forum, some time ago I bought a Smart-UPS SC 420 for my private server. It has always worked great so far. For the past two weeks, however, it is turning off the outlet group at different times every Sunday.  So, all connected servers and devices are crashed because of the power outage from the UPS.

Then, about 20 minutes later, the power is back. 

The UPS is also monitored via one of the servers with PowerChute, but there are no error messages there, and the quality of the input was great. On my servers, after booting, it only shows "The system was unexpectedly shut down at 8:18 p.m. the day before".

The battery was only replaced a few months ago, so I rule out a defect there.

Maybe one of you can help me and say, why this is happening.

Thank you!

Silash