Activity Monitor shows shortname in the Client column.
This name in Source Client of BAR must match EXACTLY.
If client was installed as client.domain, and restore is attempted from Client. this is the name that will be sent to master to request backups.
The next thing to check is the Policy Type in the backup policy.
Is it really MS-Windows?
If policy type was accidently set to Standard, the backup will work, but a search for MS-Windows will fail.
The policy type in BAR must match the type selected in the Policy.
The last thing to check is the date range (if this was selected in BAR).
Try to open BAR on the master server where you can actually select/type client name in Source Client field.
Ensure Client name an policy type matches what is in the Backup Policy config.