Not sure why this went unanswered, but if you receive error messages like this, please contact our technical support department directly for assistance. This sounds like a unique glitch, as I haven't seen this before. It could be related to an update to your software version, perhaps a port was unavailable that the local browser was attempting to use, etc.
In the mean time, there are a couple things you could try to fix this. First, go to Test | Settings... | Explore the Test Directory. Delete the test directory and then try testing the study again.
Also in the Test | Settings..., you can try stopping and restarting your Local Test Server to see if this resolves the problem.
Occasionally, the Port used in Local Test Server will be incorrect. So, when you test, it might be pointing to a server located at, say, http://127.0.0.1:12345/blahblah.html,
while the SSI Web settings (File | SSI Web Settings) are pointing to a different port.