OK - after some time and retries:

I received the new ordered HAT.
I took your Linux-Image and tried again.
Changing of the i2c Adress did not work...
What worked was to update the Firmware to the 2024.hex
After FW-Update now changing the adress works as well.
Also changing dtoverlay of the ds1307 to the adress 0x48

--> Finally after 40h and your great support I think it's running - Thank you

@ryf 

Hi,

The former HAT one must be working. We have extremely low number of faulty boards. If it is not working please send me your postal address, so I ship you new one with our costs. It is practically impossible that it is not working. We have installed tens of thousands of UPS PIco HV4.0 over the world working 24/24/7/7. If you like can send you new one (but green not red, red is not available anymore). Alternatively can send you M.2 – UPS and Power Management HAT for Raspberry Pi® 5, instead. 

Please contact me with your details on support@pimodules.com

BR PiM