Latest Firmware doesnt work on Sophos Red 15

Hello,
I just started to ReCykle Sophos Hardware. I got in trouble when I tried to install the (latest) Firmware 23.05.5
on the RED 15 Routers. The ( Basic ) Firmware installation was successful - BUT the SYSUPGRADE Image couldn't be installed.
I figured out, that the MTD (Character)device nodes were not present and therefore no access to the flash memory. The installation script
always wanted to mount and write to MTD BLOCK device - which was not working.

After several Attempts, I began to install older Releases AND succeeded with 22.03.7.
here the MTD character device nodes were present and access to the flash memory was possible.

So my question is: what has changed from 22.03.7 to (any) 13.05x release? ( None of the 23.5.x images worked out)
What can I do to Fix it?

Kind Regards and thanks in advance.

p.s.

Yesterday i got an AP 100X. Question is weather the 100X is compatible to the 100C?