Does that about match up with the amount of space you know the movies take up? Each movie could be about 1 GB. (But I'm not sure how big they actually are.) Sorry, beyond that thread in XDA I have no idea what the problem could be. Everything plays on my devices.
I had problems with Play Movies in the early days. Have you tried having auto sync enabled when attempting to watch or download a movie?
BTW, you can watch your purchased and rented films through Youtube app. Open the App, enter the side menu and select purchases (6th option down). From here you can stream all your purchased Google Play Movies content. I use this method whenever i have problems with Play Movies
I setup a player called viral lite and been watching everything on it since! I am however having trouble with my htc300 after dwnld an app then I uninstalled it but fone keeps reloading all screens after I use any app, its almost as if my fone is started to lose its mojo!! any idea's???