I'm going to make a couple of assumptions...
1) You are using the standard PLi image
2) You are trying to stream via their 'movieplayer' add-on.
I know there is an issue which as of version 3 had not been fixed. However if you download the latest stable OpenPLi image it should work ok.
To be honest, I gave up on their movieplayer plugin for the dreambox, I found it pixellated very readily but then my hardware sucks so you may get better results. If I'm totally wrong about the assumptions...sorry :)