It would appear that something bad got mixed in somewhere along the line which started affecting users of the Test Client as of the August 24 update. These users have had an ongoing working Test Client for some time before the August 24 update. It is not affecting all users, so it is hard to say when the bad stuff actually got on to your machines. It is only that the August 24 update is the one that put the bad stuff in place to start causing the problem. The bad stuff may have been there for some time before August 24, but wasn't being used.

It is very doubtful that SE has any clue at all of what that something bad is, but it is not present in files available for downloading and updates at this time.

For any Test Client users that are having this issue of POL Viewer window closing and then just reopening again after clicking the Play on Test Server button, I would recommend that you uninstall the Test Client completely, then download all files again and reinstall and update it again to get it working again.

I was hoping the problem would go away on another update subsequent to the August 24 update when it went bad. Such would seem not to be the case.