Cursor trouble when playing Private servers

[b33p]

Hello everyone,

I was just playing on a private server (Diamond) and all of a sudden my cursor starts moving up... i click outside the SRO_client window and can move my cursor again...
when i click back in the SRO_client window after a few seconds, my cursor starts moving up again.

Anyone has any idea on how to fix this?
It's only happening in SRO ... no problems with other programs, games, ...

pc config:
-Operating System: Windows 7 Ultimate 64-bit SP1
-Processor: Intel(R) Core(TM)2 Quad CPU Q8200 @ 2.33GHz (4 CPUs)
-Memory: 4096MB RAM
-DirectX Version: DirectX 11
-Video Card: NVIDIA GeForce 9600 GT (latest nvidia driver: 285.38)

Thanks in advance

Greets b33p