gvfsd-admin operation

[Zavonen]

Hello everyone,
I have a local network managed by a Free-Mini-4K box (French stuff... used mostly as a router)
On this box (USB port) is connected a large external HDD formatted in ext4.
Two Linux Mint 20 Ulya PCs are permanently connected with access to this HDD which is used for data backups and cloning (clonezilla images).
There were always problems of rights according to the PC with which one accessed the network drive but in general one could always get away with chmod and chown.
For some time I have had many access problems, sometimes the files are invisible or cannot be copied, not erased, etc. The situation varies from one user to another, from one PC to another. Anyway chmod and chown are now prohibited operations (commands fail). Switching to sudo does not change anything.
Thanks to fleeting messages appearing randomly I was able to understand that the problem was linked to gvfsd-admin or org.gtk.vfs.file-operations
There are some posts about these problems (appeared suddenly, everything worked fine for two years), but I couldn't find the solution.
If anyone can enlighten me.
Thanks in advance.
PS: disabling key-ring on start, as some claim, doesn't change anything, neither does ctrl+F2+r (cinnamon restart). It seems that I lost all my rights, even as a superuser, in favor of a certain gvfsd-admin, perhaps following an update or a bad manipulation.
Additional info : erasing .local/share/gvfs-metadata/ doesn't help. Commands found here and there to stop .gvfsd daemon do not work. And if I happen to find the correct syntax is it risky to use it ?