Does this return anything?

IF OBJECT_ID ( N'msdb.dbo.dbm_monitor_data', N'U' ) IS NULL

SELECT 'No data'

ELSE

SELECT * FROM msdb.dbo.dbm_monitor_data

or this?

SELECT

*

FROM

msdb.dbo.dbm_monitor_data

WHERE

database_id = OBJECT_ID(''NAV_Mirroring'')

Maybe there's no data to read? On my local machine these commands return null.

If I run the procedure

EXEC msdb..sp_dbmmonitorresults

@database_name = 'LocalTestDB'

,@mode = 0

,@update_table = 1

I get this error:

Msg 32039, Level 16, State 1, Procedure sp_dbmmonitorupdate, Line 99

The database 'LocalTestDB' is not being mirrored. No update of the base table was done.

I also noticed that the mirroring stats values as declared inside msdb..sp_dbmmonitorresults are TINYINT rather than INT on a few of the variables. I don't see how that would make any difference, but it's just an observation.

Sorry I can't offer any more help than this.