Time Machine doesn't show old backups and doesn't make space for new Backups

I'm using TM for years now and never had problems with it until last week.

I'm using a 850GB external HDD to backup my iMac (Late 2012) with 450GB in space use.

Since last week TM complains about not having enough space to backup my Mac.

I cannot see any of the old backups in the TM App nor with the Terminal command

tmutil listbackups

In the Finder though navigating to BackupVolume/Backups.backupdb/MyMac I can see over 100 Folders with presumably old Backups until May 2019.

I read some tips, but none of them seem to work for me.

I used the Disk Utility App to repair the drive, which had no problems. I also used the free App 'CleanMyMac X' to check for Malware.

Is there anything I can do besides erasing the the whole hard drive?