I recently upgraded my NAS operating system from Ubuntu Server 18.04 to 20.04 and suddenly my BR-20 could no longer access my files.
After many hours I worked out it was probably because the version of SAMBA (4.11) had SMB1 disabled by default. After manually editing the smb.conf file to enable NT1 I was once again able to access my files.
Perhaps I've got hold of the wrong end of the stick but, assuming that the issue was SMB1, I do wonder why MDP is still reliant on a protocol that has been deprecated for years and is probably going to be removed from Samba in the not too distant future. [Edited]