On March 20th, 2010, we noticed that many of Invisible Checker that available in web base platform got results error every time we tried to use it. It including of 80% of them. MessengeRoo Yahoo Messenger Invisible Detector also one of them that got the same error result. If someone in invisible mode the invis scanner said that they are offline.
We asked to developer of invis checker about this trouble, and they said that it because Yahoo changed something to its protocol and they need a little time to handle this trouble. Just wait for couple days and then try to check the invisible detector again. Hope its would back well soon.