I installed Joplin on a new phone (Galaxy S24 with Android 14). I couldn't sync it to my NAS with "WebDAV https://:".
I installed the total commander and its WebDAV-Plugin. Here I could access my WebDAV directories on the first trial. After this Joplin synced perfectly.
It seems the total commander initialized / fixed something. I 'm writing this down, so someone who may struggle with the same problem has a possible solution.