@myfta, while all the rest looks good to me (remember : I am not a dev), I would suggest a few more tests. If there is anything important in your profiles do backups first, then ....
a) uninstall and reinstall Joplin on both machines
b) erase profiles on both machines manually, AND on the sync target, THEN only try again
c) try syncing one machine and test, erase ALL profiles again, try syncing the other machine first
d) any difference ?
If none of this helps, try and
e) get your encryption settings right (I see error messages about that), this may seem unrelated, but since I never used Joplin without encryption, this ...
I wish I had a better response for you right now, but at this point - if the problem persists you have probably done what you can. Let's hope that one of the devs takes note of this thread and finds the time to help.