Spring RTS Engine

When I leave default resolution of the game (640x480), it runs perfent with no probles. While trying to run the game with any other resolution selected gives me this error:
pbuffer creation error wlgCreatePbufferARB() failed.
I am using WindowsXP with no service pack.

It's perhaps the shadow, try to lower/disable them.

I triedto disable them and it helped - game runs. May I ask why does it do so (drivers, not enought memory, etc??)
And another thing... why it refuses to use another resolition then 640x480 or 1024x768? When I trie it, get message:
"The Requsted wideo mode is not supported by your video card"
but I use them normaly (oh, there are "widescreen")?