Welcome to the forum @BennTech
What is your current and (if you recall) past Duplicati version, on what OS, and to what storage type?
One helpful dbconfig.json explanation is in Does the AFFECTED command need a local DB? so if it’s causing trouble and you know your database path, then you can just pass it using the –dbpath option.
The file doesn’t have much use in the GUI. AFAIK it’s for command line. Is that how you’re doing this?
The hard question is what began going wrong 3 months ago. Any idea what may have changed then? Have any logs, or other information? Going way back in time to troubleshoot is frequently not possible.
If storage permits this, sometimes people start a new backup just to have a backup running while they see what can be made of the old backup. Using another system is best because attempts take awhile. There are a finite number of tools to use, and you’ve already used some of them but perhaps not all…
A general precaution is to keep backups of the DB (and maybe also backup files) while experimenting.