A while ago i had set up file system synchronization to backup my notes. It didn't work that well (tried to backup into a mounted NAS directory), but whenever it failed i got the fail-save error message that prevented the (empty) remote directory to overwrite my files. Until today. I created two more notes and then shutdown my PC. When i booted it up later, the two notes were still there, but everything else was gone. I traced the logfile and it seems to me that for some reason the backup sync decided to actually overwrite my local files with the empty remote directory this time. I checked my config, but the checkbox preventing an empty overwrite is still set. So my question is: Is there anything i can do to retrieve all the notes i collected in the last 18 months or are they gone forever?
Current versions of Joplin have a backup plugin build into them, which is enabled by default, making it possible to restore accidentally deleted notes. The problem is that your Joplin version is heavily outdated, and judging by the posted information, you've got no plugins installed whatsoever, so unfortunately, I don't think you can restore the notes (unless you go into data recovery and manage to recover actual MD files, but even then, unless you restore literally everything in its original state, you will likely need to open them in a text editor and copy the content manually).
It took a while, but i was able to recover 98% of my notes! Thank you very much everybody, who suggested methods and especially @personalizedrefriger. The DB recovery log contained most of the missing pieces. I also updated Joplin to the newest version and inserted some extra backup steps to prevent something like this in the future. Again, thank you to everyone!
(unless you go into data recovery and manage to recover actual MD files, but even then, unless you restore literally everything in its original state, you will likely need to open them in a text editor and copy the content manually).