I suddenly found that I was getting "Windows Media Player cannot play this DVD because a problem occurred with digital copyright protection." errors when trying to play DVD's with Windows Media Player 9. I had previously been able to play DVDs with no problems.
I turned out that this was as a result of upgrading to Nero 18.104.22.168 from 22.214.171.124. If I reinstall the 126.96.36.199, I am able to play DVDs again. I upgraded because when I first tried using 188.8.131.52 a message popped up saying that I needed to upgrade because this version was incompatible with my version of Windows (or words to that effect). I am on Windows XP SP2.
I did find that 'Media Player Classic' will still play the DVDs with Nero 184.108.40.206.
I can see no reason why the upgrade should stop me playing DVD's. Is there a new setting on Nero that I need to change?
I suspect that other people have hit this problem as there are similar reports of suddenly not being able to play DVDs on other message boards.