It sounds like the “Repair cannot acquire…” issue is a known logic loophole that apparently hasn’t been fixed - what version of Duplicati are you using?
The link above suggests that if you remove (move to another folder or delete) the “broken” dindex file then a database Repair should rebuild it correctly.
The trick is, I don’t know how to tell which dindex file goes with which dblock (or maybe @Pectojin has already shared it and I’ve forgotten) . 
If this was one of my less important backups I would try MOVING all the dindex files with the same date as the problem dblock file to another folder then run a database Rebuild which should regenerate the missing dindex files. If it worked then great, but if not I’d put all the original dindex files back. But that’s just me, and I’m a little crazy… 