Backup disk becomes unavailable after each backup

Hi,

I have been experiencing peculiar Time Machine problems lately:

- As my TM Backup was not starting due to "backup disk is not available" (although the disk is mounted and readable in Finder", I reset the TM (deleting pref files, reselcting and making options again...)

- Backup then works fine, maintaining the existing backup archive and adding on it as usual

- but then the following (auto or manual) backup fails, mentionning the disk is (again not available). For each I have to reset and reselect all the time.

Has anyone experienced the same and found a fix? I check the Pondini help webage, and after doing the TM full reset, still have the same failure on second attempt 😟

Thanks for any tips