This are my findings for an issue I was not able to resolve yet, but with this article I am going to write down my findings and hopefully this helps me to find a way to get rid of it. This problem popped up some time ago, it is not happening with every backup which is done, but happens more frequently the last few weeks.
TLDR; After spending quite some time testing and figuring out the root cause, we did not succeed in finding it. The good news: We fixed the backup at the end. We found the broken dblock files, we were able to verify which files affected by the broken dblock files and in my case they were still on my filesystem. This allowed me to use the Disaster Recovery Guide. I deleted the affected dblock files from the backup set, ran the
purge-broken-files command and after many hours and days I was able to start a normal backup. Because the files were still on the filesytem, affected by the broken files, they got re-added to the backup. Thanks to ts678 for the help.
There is a discussion on the duplicati forum which is the reason I am creating this article to keep an overview.