After a week of uninterrupted mail retrieval it is safe to say that this solution worked ;-) Tjanks

This setting was off in all backup plans. Will let you know the results...

I'm backing up to Amazon Glacier. Yes, I looked at files.

Thanks for the quick reply. There is no "Generate detailed report" option on the Notification step of the Backup Wizard, though:

https://i.imgur.com/ax1XC0q.png

I dove into the backup history and the only 'Failed' operations are FireFox cache entries (files).