p2os serial connection error

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I am trying to connect to a Amigobot (similar to Pioneer) using the p2os stack. I am using an USB to Serial Converter in Ubuntu 10.11. I used rosparam set p2os/port /dev/ttyUSB0 to set the port to USB0. Unfortunatetly when I try to connect I get this error:

roslaunch p2os_launch p2os_driver.launch
... logging to /home/mario/.ros/log/cdf4ba1a-0bb0-11e2-8b6f-37e053dcb6c9/roslaunch-mario-Satellite-C660-12071.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://mario-Satellite-C660:56326/

SUMMARY
========

PARAMETERS
 * /rosversion
 * /rosdistro

NODES
  /
    p2os (p2os_driver/p2os)

ROS_MASTER_URI=http://localhost:11311

core service [/rosout] found
process[p2os-1]: started with pid [12089]
[ERROR] [1349087058.764888277]: Error reading packet header from robot connection: P2OSPacket():Receive():read():
.
.
.
[ERROR] [1349087114.446749752]: Error reading packet header from robot connection: P2OSPacket():Receive():read():
[ERROR] [1349087114.447846009]: p2os setup failed...
[p2os-1] process has died [pid 12089, exit code 255].
log files: /home/mario/.ros/log/cdf4ba1a-0bb0-11e2-8b6f-37e053dcb6c9/p2os-1*.log

The output of the log file looks like this

^[[0m[ INFO] [1349093885.560391832]: using serial port: [/dev/ttyUSB0]^[[0m
^[[0m[ INFO] [1349093885.637932732]: P2OS connection opening serial port /dev/ttyUSB0...^[[0m

Maybe the baud rate is not correct? Do I have to set it manually?