Please look at the last couple of pages of the YouTube Add-on thread, where you can get access to a newer and fully working version of the add-on. The current one in the official repo is rather broken as its maintainer has stepped back from the project and there are some problems subsequently with it.
Hopefully one of the new version authors will step up and allow the repo version to be updated and restored properly, but in the meantime you can download and install from zip via the links in the posts near the end of the thread.