I can't run rviz

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Hi, I am trying to use rviz, but recently I receive the following error:

[ INFO] [1328487732.257914711]: rviz revision number 1.6.7
[ INFO] [1328487732.258118438]: ogre_tools revision number 1.6.2
[ INFO] [1328487732.258162578]: compiled against OGRE version 1.7.3 (Cthugha)
[ INFO] [1328487732.492394503]: Loading general config from [/home/jabrena/.rviz/config]
[ INFO] [1328487732.493823666]: Loading display config from [/home/jabrena/.rviz/display_config]
[ INFO] [1328487732.598258030]: RTT Preferred Mode is PBuffer.
do_wait: drmWaitVBlank returned -1, IRQs don't seem to be working correctly.
Try adjusting the vblank_mode configuration parameter.
[ INFO] [1328487733.190865822]: Texture for pass 0: creating with size 1 x 1
Segmentation fault

How to modify the parameter vblank_mode? What is the right value?

Many thanks in advance.

Juan Antonio