Are you able to play local videos? I saw a post once on this very same phone but he couldn't get any kind of video to play. I don't think I ever saw a solution though...
EDIT: I found this on that other thread... hope it helps.
I have the HTC one and ran into this problem yesterday. The problem seems to be that Google Play Movies cannot communicate with the device properly. Once you try to play anything in google play, your videos no longer work. In order to fix this go into your settings > Apps > swipe over to all. Now go through all of your apps that have anything to to with video; vine, youtube, Etc. and clear the cache on all of these apps (make sure you hit Google Play Movies and View Video). Next go to the Play Store and uninstall all updates to Google Play Movies, then reinstall. After all of this do a simulated battery pull (volume down + lock) and that should do the trick, worked for me anyways. I tried going into Play Movies again and had to repeat the process. Hope this works for you too!