I'm aware of the problem concerning watching some of the old videos. I don't know why some of the older videos don't work anymore.
I've already sent an e-mail to the coder of the embedded video player we're using in our blog. He's looking into it as well, he's still investigating the issue. Hopefully soon it'll get fixed.
If the problem persists, we're planning to try new methods. So do not panic.
In meantime you can watch the 'problematic' episodes using the alternative way which was already explained here.
UPDATE (Feb 1st):
"It appears to be a problem on YouTube's side, people are complaining about this on youtube/google's dev forums (me included), so I think they'll fix it soon," said the coder of the embedded video player we use in our pages.
And he added, "Ok, after a bit of research I found out the reason for this. It is the following update by YouTube , in which they are pulling AS2 Youtube Chromeless API support for 'certain videos' - those videos being those they would like to monetize by showing ads, which is not possible for YouTube in a chromeless AS2 player. Overstream player is a legacy one written in AS2 (ActionScript 2) and uses the Youtube Chromeless API, therefore it was affected by this. To make this problem go away, I will either need to find a suitable AS3 player or use the regular YouTube AS2 player (with their buttons). Even so I am not sure what things will look like when the subtitles would appear on top of an ad..."
This means, we have to be patient and wait while it is -hopefully- getting fixed. Or use the alternative methods.
You'll be updated. Stay tuned.