Force sync doesn't sync new files

Using Nextcloud client 3.7.1 (but also earlier versions) on Linux Debian.
In general, Nextcloud sync works, but very often, it simply won’t sync new or modified files (usually, it will not download files from the server).
Worst of all is that even after running “Force sync now”, Nextcloud will show a green tick mark but the folders are still not synced.

My only workaround is to create an empty file on the local machine, inside the folder that did not sync. Only then will Nextcloud agree to sync it.

The logfile doesn’t give any relevant message.

Any idea what’s going on?