I have a Mac mini running 10.14.5 as a server. In System Preferences / Sharing I set up a shared folder on second internal drive HFS+ formatted drive for time machine backups. Backing up and entering the Time Machine backups works fine from three different clients all running 10.14.5. All backups are encrypted. On the server I can mount the sparsebundle  and read the backup (so I have the right key).

Now I replaced one client and want to restore a backup. During the setup I can connect with my file server user to the server and see all three clients, but when I select one of the clients, I get the message "some backups cannot be opened". I never get asked for the encryption key.

Following are my settings:

 - File Sharing: ON at "smb://server.local"
 - Share files using SMB: ON
 - Share files using AFP: OFF
 - Shared Folder Advance: Share as Time Machine destination on, Share over SMB, only encrypted SMB connections OFF
 - Shared Folder Permissions: two user can read and write, admin & everyone can read