This is a known issue and has been fixed in version 12.55. There is a manual workaround for 12.50.
The workaround is:

1. Log into My Performance Center
2. Confirm Auto Refresh is off
3. Under test resources create a new profile
4. Within that profile add a SiteScope monitor
5. Counters are loaded, select any counter and hit save
6. Monitor is added when auto refresh is off