Thank you for your sugestion. I forgot that i fooled around with the developement repo didn’t remove it. Now i removed it from the sources.list file and made the following downgrade and everything works again. Thank you very much!
osmc@osmc:~$ sudo apt-get install rbp2-libcec-osmc=4.0.4-5 --reinstall
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be DOWNGRADED:
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Need to get 310 kB of archives.
After this operation, 18.4 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://ftp.fau.de/osmc/osmc/apt buster/main armhf rbp2-libcec-osmc armhf 4.0.4-5 [310 kB]
Fetched 310 kB in 1s (544 kB/s)
dpkg: warning: downgrading rbp2-libcec-osmc from 6.0.2-3 to 4.0.4-5
(Reading database ... 49721 files and directories currently installed.)
Preparing to unpack .../rbp2-libcec-osmc_4.0.4-5_armhf.deb ...
Unpacking rbp2-libcec-osmc (4.0.4-5) over (6.0.2-3) ...
Setting up rbp2-libcec-osmc (4.0.4-5) ...