@Addle - Unter BigSur you may still use Mac OS Extended (Journaled) as TM files system. No problems then. On Monterey, you unfortunately might no longer add a non AFPS case-sensitive (adding a pre-formatted HFS+ disk to the list of TM disks, Monterey forces it into a APFS case-sensitive, independent if you check "Encrypted" or not. PS, FYC, @Apple: there are many file system/Finder issues in Monterey. Even Quick View does not work as expected (e.g. chapters in MOVs not shown).

@Pixalab - how did you fix it?

@jkrosado - didn't fix my (same) problem: Running First Aid on “TM8TMB16” (disk3s2) Checking file system and repairing if necessary and if possible. Volume was successfully unmounted. Performing fsck_apfs -y -x /dev/rdisk3s2 Checking the container superblock. Checking the space manager. error: (oid 0x7013) cib: found zeroed-out block error: failed to read spaceman cib 2 at address 0x7013 Space manager is invalid. The volume /dev/rdisk3s2 could not be verified completely. File system check exit code is 8. Restoring the original state found as mounted. Problem -69842 occurred while restoring the original mount state. File system verify or repair failed. : (-69845) Operation failed… Then trying to mount ends by error message "Could not mount “TM8TMB16”. (com.apple.DiskManagement.disenter error 49218.)"

Unfortunately not for me

Not for me. This is most probably (99.99%) an issue to be fixed by Apple’s macOS developers.

That does actually fix nothing. Next time you unmount and try to mount again, you face most probably the same problem.