This seems to be the solution (at least for now):
https://www.reddit.com/r/tuxedocomputers/comments/1f8kc1j/kdeconnect_suddenly_stopped_working_gives_errors/llffq38/?context=3
[I’m not allowed to do proper links for some reason]
Although, I still have no idea why this bug suddenly occurred.
I also added a comment on this bug here:
https://bugs.kde.org/show_bug.cgi?id=489343#c4
which seems to be the same thing