I've been getting the "Sorry... This activity is no longer available" message constantly for several days now, with no videos I can actually watch.
I'm really confused by this, but it appears that sometimes, when you finish watching video #1 and it offers you a link to continue to the next video you're eligible to watch, no new window appears when you click but the sound for a nonexistent video starts playing after a minute or two anyway. I still seem to get GC for these invisible videos; there just seems to be a glitch somewhere along the line that probably should be looked at.
This has been happening for the last two days. Before, if there weren't any videos that I was eligible to watch, it simply wouldn't show me anything. The weird thing is that these are videos that I've watched multiple times before (once a day or so) successfully.