These days I've been lazy so I often rent a video from youtube.com instead of going to rental shops.
Usually I can watch a movie after purchasing but this time I could not. I guess this is because of using Ubuntu OS this time so then I googled.
As a result, fortunately I found the solution for this problem. In a word, the problem was the environment on Ubuntu OS could not allow to stream a protected movie. So now only you have to do is just to install a necessary hardware and remove some cache to solve this problem.
There are just only 3 steps.
On the terminal:
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sudo apt-get install hal
cd ~/.adobe/Flash_Player
rm -rf NativeCache AssetCache APSPrivateData2
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That's all!
I guess after restarting your browser, you can watch a movie successfully.