As the Joplin server can be installed multiple times (keeping resource usage in mind), does installing another copy work?
You could then look up the yarn location, fix this installation and then remove the other one again.
Last week(ish?) I tried and installed Joplin server with no problem (although it takes a while), so if it does not work anymore it is a recent “development”.
Speaking of recent developments, Orhtej2 has been packing an optimized version that uses fewer resources.
Having both of you using this server, does it provide any benefits over using plain WebDAV or Nextcloud for syncing? When I had it installed, there did not seem to be any editing or viewing options (but I’m not a Joplin user, so I may not have recognized it).