Zircon parallel batteries

[DD86]

Greetings,
My Zircon AX-1000 UPS with standard 12V 7.8Ah battery expanded with the same type of battery. After a short time the entire battery function stops and does not recover when the second battery is removed.
A previous Zircon 'Smooth-D' that I had did not cause any problems.
A third Zircon UPS I apparently also messed up by connecting batteries in parallel.
Now I don't have much use for it other than possibly a surge protector.

Zircon in this country indicates that they want to check/repair it and that I can send the UPS. But then the UPS is repaired by default and I have the same situation again, with an empty space in my wallet.
Does anyone have experience with the situation as described? There is certainly a solution, but in this country it is a lot more difficult due to language problems.
Thanks in advance