YOUTUBE TEMPORARY SERVICE DISRUPTION
It has been noticed, since latest server TV firmware update, that some 2017 Smart TVs have been experiencing YouTube service disruption (only webOS 3.5 models appear to have been affected). We wholeheartedly apologize for the inconvenience. LG software engineers are working to resolve this issue as quickly as deemed possible in the hope to get it back running within next few days.
There is no guarantee the following will resolve your issue in the interim time but it is worth the try. Proceed by un-pairing Magic Motion Remote Controller to television then pairing it back.
Again, no guarantee it will work with all models. If not, you will need to wait for next server TV firmware update which should be made available any day soon.