Time Machine not working since upgrade to High Sierra

I recently upgraded my 2012 MBPro to High Sierra. Everything seems to work well, but I can no longer get Time Machine to work on the very same HDD that I have always been using for TM backups.

The error appears immediately when the backup process starts and the error message looks as follows. This translates as "Time Machine cannot complete the backup on 'TM' (this is the name of the volume). An error has occured when switching on ownership rights on the backup drive. You possibly need to repair or wipe the disk with disk utility. If the disk cannot be repaired, use a different disk.'

So far I tried the following, but all to no avail:

• Using two different external USB HDDs (copying files from/to these works fine) and the same error appears with both drives.
• Wiping / formatting one of the external HDDs again (as MacOS Journaled, GUID)
• Switching off Time Machine, rebooting, trying TM again.
• Using different user accounts with/without admin privileges.

The ignore ownership details checkbox at the bottom of the info pane of the Volume is ticked.

If I try to untick this box (after unlocking with admin password) I also get an error, which translates as 'this action cannot be completed. An unknown error has occured (error code -50)' :

Console shows the following:

Any suggestions?

Thank you!