I think I found the problem.
The command to mount the ADMIN$ shared folder is not the correct one for Windows 10. It must be in version 3.0
Command in failure:
# mount -t cifs -o username=XXX,password=XXX,domain=XXX //poste_xxx.protection24.lan/ADMIN$ /tmp/testdir/
mount error(95): Operation not supported
Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)
Successful Command:
# mount -t cifs -o username=XXX,password=XXX,domain=XXX,vers=3.0 //poste_xxx.protection24.lan/ADMIN$ /tmp/testdir/
Is there a setting in Remote Administrator to change the version following the computers?