That happened to me yesterday and I had to return everything I had in my queue because I couldn't get any of them to work. I found this info on it:
Yes, those errors happen sometimes from Google-affiliated requesters like these.
- If you return one of their HITs, you'll get that error on any future HITs from them until the time when the returned HIT would've expired.
- If you accept more than one of their HITs simultaneously, you'll get that error on all but one of them (look through your queue and find the working one); then after you complete that one, another one will become the working one.
- And you'll get the error sometimes 'just because', even when you aren't in one of those situations; in that case, right-click inside the HIT iframe and select 'Reload Frame', and it will usually work then.