I found the solution it is in Settings - Websites - Full screen - Manage Exceptions. Just delete the blockage of the full screen on Youtube or whatever website you have put it on.

I have similar problem.
I used to be able to open videos in Youtube in full screen mode with Opera.
Yesterday I was watching a yYoutube video and there was a pop-up on top of the screen asking me would I like current preferences for the web site to be remembered. I chose them to be remembered while the video was in a normal (not full screen) mode. Now I can not view any video in Youtube in a full screen mode. When I click on the full screen tab in the lower right corner of the video nothing happens. How can I rectify this problem? How can I change my settings for the youtube web site? I can view videos in full screen on other websites but not on youtube.