If you do have a USB drive connected see how the TM rates it after a format, mine rates my 8gb Verbatim pen drive as low performance. I figure this is why You Tube doesn't buffer hence doesn't work.
A high performing drive will probably play fine.
I do recall though that some versions of the TM had problems with You tube. For me personally I wouldn't worry about You tube as the TM version is far to restrictive in any case to be enjoyed.