For the last few months, Google has been encouraging us to fully merge our YouTube and Google accounts, presumably so things can get more integrated with Google Plus and whatnot. I’ve resisted this as long as I can, but it’s no longer possible to do that and still use the YouTube add-on for XBMC.
The background to this is that YouTube has maintained two accounts under your Google login – your Google account and the old YouTube account. This means all your subscriptions, favourites etc. Are not shared between the two. Until recently, the default account used when you logged in was the old YouTube account. This was fine for XBMC since it looked the same as it always did. Now though, the default account is your Google account, so the YouTube add-on in XBMC no longer sees your subscriptions.
The solution to this is to transfer all your subscriptions, favourites etc. From your original account to your Google account. Sadly there’s no way to do this automatically – you have to re-subscribe to your channels manually all over again.
Presumably at some point Google intends to merge our parallel accounts to remove this problem. Until then, the joy of manual migration is yours to enjoy. But it will mean you can use XBMC properly again, so it’s worth it.