You can see the errors when you run the saved search "Recent DB Errors".
index=_internal sourcetype=dbx_debug severity=ERROR OR severity=FATAL
I keep getting one of the following errors as I use a custom SQL and I think the filter is not working correctly or at least the format is not documented correctly:
Configuration Error: Invalid query specified! Missing placeholder for condition!
Error while executing database monitor: java.sql.SQLSyntaxErrorException: ORA-00936: missing expression