The kernel configuration was the trick!

[Milosch Meriac]

Dear Clemens,

Clemens Gruber wrote:
> We don't understand what's going wrong.
> I hope somebody can help us..

Probably SVN is currently broken. Please try r314 for OpenPCD firmware 
and r1999 for librfid. Please make sure to use the same librfid release 
for the firmware and for the host libraries. These two releases work 
well for sure.

If you find time it would be great to figure out which release 
introduced the broken code - i will remove this particular patch. Don't 
hesitate to ask if you have any further questions.

Warmest regards,
--
Milosch