Hi grt.c...,
On the web UI click the YouTube tab and enter a search clue such as 'Get it on' and click the spyglass. Click on one of the results and it should play after a few seconds.
If this doesn't happen, close the web UI page and go to Settings for the browser. Clear browser data - select 'cached images and files' for Chrome, etc and 'Delete History and website date' for Safari. Open a new browser page and enter the IP manually, rather than using a bookmark. Try YouTube again.
Paul
Brennan Support.