All the problems and solutions step by step here:
http://dri.freedesktop.org/wiki/DriTroubleshooting
The problem in my case was:
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If the driver complains about unresolved symbols:
libGL error: dlopen /usr/X11R6/lib/modules/dri/radeon_dri.so failed
(/usr/X11R6/lib/modules/dri/radeon_dri.so: undefined symbol: _glapi_noop_enable_warnings)
then your libGL is out of sync with your DRI drivers.
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By running:
LIBGL_DEBUG=verbose glxinfo
I could see that error and the directory where libGL searches for the drivers. It was:
/usr/X11R6/lib/modules/dri/
(and the radeon_dri.so in it)
When I start Debian in fail save mode, I have a working 3D and libGL searchs the following directory for the radeon_dri.so module:
/usr/lib/dri
According to the DRITroubleshooting guide, libGL must work with a specific version of the drivers, usually latest. So I've deleted the drivers in /usr/X11R6/lib/modules/dri/:
ffb_dri.so i830_dri.so mach64_dri.so r128_dri.so r300_dri.so s3v_dri.so sis_dri.so trident_dri.so
i810_dri.so i915_dri.so mga_dri.so r200_dri.so radeon_dri.so savage_dri.so tdfx_dri.so unichrome_dri.so
And copied those from /usr/lib/dri to /usr/X11R6/lib/modules/dri/. After restart of the X server glxinfo reported "direct rendering: Yes" and my 3D is working now