My problem looks very similar to this thread Nextcloud : liens home et multimedia KO
[core] Error: Following symlinks is not allowed ('/home/yunohost.multimedia/myuser//Share' -> '/home/yunohost.multimedia/share/' not inside '/home/yunohost.multimedia/myuser/')
This Share symlink was present in each $user folder. Based on the error code, I have deleted all offending symlinks because I do not use them. This sort of solves the problem … but I don’t know where they come from (Nextcloud, Yunohost or Debian) and why this suddenly stopped working.