When I first tried to connect through OpenGL mode I always encountered this error. Then I looked at some forums and decided to do this.
I opened up the file \"psclient.cfg\" using the program \"Notepad\", and there should be a code within that file, like this:
;Video.FullScreen = no
I changed it to:
Video.FullScreen = yes
When I connected to Planeshift through OpenGL mode. it worked! I think this may be the key to making the OpenGL mode to function properly.