I tried to race into the programm with GDB, to understand where the
> problem arise, but I cannot trace beyond a function call which is
> defined in a shared library: impossible to set a breakpoint in the
> library, since GDB tells me it knows nothing about the source files of
> the shared library. I tried the directory command, but nothing changes.
> 
> Any hint?

With GDB, two solution:

1) Rebuild the application as static. All thoses shared libs are a pain.

2) Wait for the shared library to be loaded. To do this, just break
before entering the function which is in the shared lib. At that time,
the shared lib is loaded, and you can set the breakpoint in the function
inside the shared lib.

This can be made automatic, by making a gdb script: make a file cmd.gdb
with this:
break before_entering
run
break inside_the_function
continue

and then use the "source cmd.gdb" command in gdb.