Okay, maybe some additional piece of information. I observed some different behavior, this time no error message but still sync does not seem to work.
Linux/Appimage 1.0.242: I wiped the settings folder again, starting fresh. For synchronization, I enter an empty folder on Nextcloud. The “check synchronization configuration” button works, gives positive result ("…appears to be correct").
Joplin starts synchronization: makes a folder called “locks” on the Nextcloud. The synchronization indicator keeps spinning. It creates an "exclusive_desktop_…json file. What I didn’t notice before is that this file is uploaded again and again in a loop (about 10 times in a minute).
Sync cancel button doesn’t work.
Killing the application and restarting shows the notes. When pushing the sync button, it starts uploading the exclusive_desktop file again and again. This time no error message occurs (at least for ~15 minutes), but syncing is also not done.