Ubuntu Dual Monitor - Stuck Mouse

I have a multi-monitor setup, but my mouse on C1 get's stuck at some height and won't go lower. Making C2 completely useless. I guess this is a problem because C1 and C2 don't have the same height, but how can I fix that?

My Setup:

Server: Win8 64bit
Client: Ubuntu 13.04

+--------+ +-------------+
| C1     | |    Server   |
|        | |             |
+--------+ +-------------+
+--------+
| C2     |
+--------+