Ok, I found the way to force it:
- open a nautilus window (change from home to another directory)
- open a second nautilus window (don't change directory)
- resize the second window
- click on a file in the second window and move it -> crash

I don't know if the directory changes are required. I tried it three times now, two times I had to hard shut down (by pressing the power button 4 secs), one time the x-server crashed and restarted automatically ("3. time", see below).

This happend (syslog):
1. time:
May 11 10:55:03 fb-ubuntu kernel: [  763.728257] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:03 fb-ubuntu kernel: [  763.728649] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:07 fb-ubuntu kernel: [  767.756671] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:07 fb-ubuntu kernel: [  767.757073] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:11 fb-ubuntu kernel: [  771.779804] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:11 fb-ubuntu kernel: [  771.780203] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:15 fb-ubuntu kernel: [  775.802202] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 10:55:15 fb-ubuntu kernel: [  775.802603] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023

2. time:
May 11 11:10:47 fb-ubuntu kernel: [  160.081420] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:10:47 fb-ubuntu kernel: [  160.081848] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:10:51 fb-ubuntu kernel: [  164.105384] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:10:51 fb-ubuntu kernel: [  164.105788] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:10:55 fb-ubuntu kernel: [  168.101674] eth1: No ProbeResp from current AP 00:14:bf:c9:1f:a3 - assume out of range
May 11 11:11:00 fb-ubuntu NetworkManager: <info>  Supplicant state changed: 0 
May 11 11:11:00 fb-ubuntu kernel: [  168.122305] NVRM: Xid (0001:00): 13, 0003 00000000 00008297 00001458 00001207 00000040
May 11 11:11:00 fb-ubuntu kernel: [  168.122785] NVRM: Xid (0001:00): 13, 0003 00000000 00008297 00001458 00001207 00000000

3. time:
May 11 11:19:34 fb-ubuntu kernel: [  144.032561] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:19:34 fb-ubuntu kernel: [  144.032969] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:19:38 fb-ubuntu gdm[6021]: WARNING: gdm_slave_xioerror_handler: Schwerwiegender X-Fehler - :0 wird neu gestartet 


For "3. time" here's the corresponding part in kern.log:
May 11 11:19:34 fb-ubuntu kernel: [  144.032561] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023
May 11 11:19:34 fb-ubuntu kernel: [  144.032969] NVRM: Xid (0001:00): 13, 0001 00000000 00005039 00000328 00000000 00000023


If whole logs are required please do not hesitate to ask for them.

It would be really nice if somebody could try the procedure I described at the beginning of this post to track down this bug.