I think I've been blocked by Research Tasks. I haven't done any work for them in a while. I did look at a couple of their video chat HITs and returned them because I thought they were underpaid and didn't work well for me. Now I see I am not qualled to do them.
If they blocked me for returning 3-4 HITs, then that's absolutely ridiculous.
EDIT: I no longer have the "Auto-Granted Qualification to Prevent too many Retakes is 1" because the author deactivated it. So either it's global on their part or a block for me for absolutely nothing.