If you attempt to monitor in SQL Server single-user mode when the Transaction Log is running, the following error occurs in the DPA logs:
Monitor for database name failed in job [QuickPollJob] due to [Database name is already open and can only have one user at a time.
When this error occurs, it shuts down the DPA and repeatedly attempts to restart, which is caused by an issue SQL Server has with running the monitoring query at the same time as the dbcc sqlperf('logspace') query if one or more databases in the monitored instance are in single-user mode.
The dbcc command is used by the Transaction Log Freespace alert in DPA. If the dbcc command takes a long time to run or is run often (again, if there is also a database in single-user mode), the DPA monitor will not have continuous monitoring.
All versions of DPA
To allow the DPA to monitor instances with single-user mode databases without this error, ensure that the monitored db instance is not in the Selected Database Instances list for a Transaction Log Freespace alert.