MacOS update installer fails: OS15.1 on iMac13,2

I have had 15.0 and 15.1 running without problems on my iMac13,2 (27” late 2012, 24GB, 2TB ssd). When 15.1.1 was released that changed. Probably the system prepared for the update and could not boot anymore. Finally I erased the ssd. I prepared an installer on my MacBook6,1 for host 13,2. However ‘An error occurred preparing the software update’. I checked the system by installing Mountain Lion. It runs without issues (except that it does not support https so it’s a dead end). Oh, I also lifted a lot of SIP restrictions, to no avail.

Any suggestions on how to proceed are most welcome, as I miss my great old iMac!