Not the author, but from the description, you can only request one check at a time. When you get the results of your current check back, you get a new token.

I'm guessing that the server got too busy to serve a request, either the last request, or any previous one, so that you broke the token chain. Another possibility could be a bug with the timing of events, such that it doesn't realize the field has changed.

The author seems to be lurking around here, maybe he'll have a better idea.