Update: There doesn't seem to be any problems with Ubuntu 14.04 yet...
The latest problem is that flash in youtube is very slow and unstable. The problem persists in Chrome too, which is my main browser. Fixing a friends computer I found the solution.
The problem is that there are two flash players installes, one is 11.8.800.96, while the other is 11.2 r202. It seems that you have to disable the 11.2 version manually.
You have to write chrome://plugins in the chrome address bar, then click on details in the top right corner and disable it...
Unfortunately, the above solution was not working well. This is what really solved my problems:
sudo apt-get remove --purge adobe-flashplugin flashplugin* nspluginwrapper
sudo apt-get install --reinstall adobe-flashplugin