So far I wasn't able to watch 2 YouTube videos at some point. It does work if I open an anonymous window and load the video there. The browser is Opera.
E.g. this video loads until 1:00
For this specific video, if I jump to a later point, then that point can be revisited, but it kind of bugs the frames. There's another video I saw earlier with a similiar issue, but I guess that's all I need to include here.
Is this some cookies issue?
About off-topic: I asked this in webapps.stackexchange.com earlier and they closed it as off-topic because it's about problem troubleshooting and they recommended me to ask it in superuser.com.