APC Back-UPS ES 700VA, beeping and outputs shut down

What could be a problem when that ES 700 VA UPS started to cut off it's output voltages from upper and lower schuko-connector banks, and start to beep continuously at same time? While it's boot sequences end it starts to make some low level noise as something needs additional amount of current, or similar. Then some relay latches, beeping starts and outputs shuts down. Green pwr-led shuts down too and Maaster-slave -system won't work neither (that led do not dimm at all). That happens similarly next starting time, it won't stop that behaviour.

(Quick test showed that: when feeding itself from it's master-connector it starts and stays on pretending that all is ok, but it wont last for example small soldering station current, etc..).

It has been serving us 2-3 years feeding some networking and computers, from cooler place than normal living room.

It's battery is not guilty, I have tested with two batteries (good 12 Ah and brand new 9 Ah, both fully charged before installing into UPS rear slot) instead of that original one, but nothing changes.

I tried to use it now at my HF radioamateur station (with a lan switch, several computers and other ham shack equipments, etc) so additional RF fields might has be near (especially when using FT8-mode) it, but max total ampere rating drawn from it has not exceeded 10 A (230 VAC).

Any ideas where to look first? Before throwing it to CER and replacing that with some other one..

Regards, Kari