Long beep, stop, then another long beep, no LED indicator

[ChickenBeast]

Hello!

I bought an APC Back-UPS 850 on 1 July, and I have a problem with it where after resetting the circuit breaker in my home after it was tripped, and I turn on the UPS by pressing the button for a couple of seconds, it just gives a long beep, stops, then another long beep that won't stop, and occasionally the LED blinks red a couple of times, but this doesn't always happen.

What was connected to the UPS:
- PC, network router, IP camera, LED lamp, all through a power s trip

- Network modem
- 2 monitors
- NAS

What I already tried:
- Unplugging it from the power source
- Disconnecting every connected device
- Pressing the circuit breaker reset button on the UPS
- Disconnecting and reconnecting the battery
- While the battery was disconnected, I pressed the power button in hope it would reset it

But all with no progress. After many tries it somehow turns on, but I think it shouldn't turn on just by chance. And I had a similar problem with my BE700G-GR as well, that just wouldn't turn on at all. And this particular beeping with no LED indicator isn't in the manual either, so I don't know what to do.

And the same happens when I turn off the UPS manually, and I turn back on after some time. The only difference is that in that case the LED blinks green, and the battery status green LEDs are also visible, but after like 5 seconds, it turns off automatically, and just beeps.

Model number: BE850G2-GR
Serial number: 5B2145T78119

Is there anything I could do to fix this?