As a Joplin user, I suggest you backup your central repository (by export all notes from yor Kubuntu desktop) before doing anything. Then delete the Joplin profile directory (Joplin Database and Profile Location) and do a fresh install, and do a full sync. Hope it helps.
I'm using both macos and windows clients. I can't remember exactly the behavior of the full sync of the windows client, but I remember it took much longer than 15 minutes to fetch everything of the 300MB remote repo to local.