[Milhouse]

and reinstalling the youtube plugin (with deletion of user data) works for me.

Since #0706 (PR12277) the version for settings changed from 1 to 2, and this causes issues when downgrading as your addon settings (now written with the latest version, ie. 2) will not be compatible with older builds that are expecting version 1, and so you find that addons can no longer read your old settings and either use defaults (if you're lucky) or fail in weird and marvellous new ways so that deleting all your settings is probably the only sane solution. As you can imagine, this is making regression testing even more fun than usual.

Edit: Without a debug log I don't know what the original problem with YouTube is, but it's probably settings related and you should contact the addon maintainer.