It looks as though the SSL cert on the server hosting/mirroring the OSMC repo expired this morning.
Err:4 https://ftp.fau.de/osmc/osmc/apt bullseye InRelease
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 131.188.12.211 443]
W: Failed to fetch http://apt.osmc.tv/dists/bullseye/InRelease Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 131.188.12.211 443]
W: Some index files failed to download. They have been ignored, or old ones used instead.