I am getting the same message. Or simply “unavailable”. Despite being able to access the YouTube items perfectly on my ipad. Seems to happen mostly on music videos. Factual items, e.g. Fred Dibnah videos, are usually OK.
I seem to remember seeing a message popping up telling me that YouTube was now using https and that certain Humax products would be unable to access the videos. I don’t think the 1800T/2000T was one of the products mentioned. However, I couldn’t access any items with my 2000T. Chances are YT have messed things up and it would require a firmware update from Humax to sort out the problems.