Flash stopped working in Safari

Flash stopped working in Safari

There's tons of threads on this but I can't find a solution that works
(e.g. this here). When using Safari, Flash is not working on any website.
Youtube e.g. recommends to install the newest version (which it of course
is already). Flash works without problems in Firefox though.
I'm using Safari 6.0.5 and Flash 11.8.800.94 is installed.
I tried
de/reinstalling
Deleting the plugin files in /Library/Internet\ Plug-Ins
Deinstalling, restarting then reinstalling
Disabling all extensions (Ghostery, Clickto, etc)