assemble() won't work with cross() in variational formulation

Hello I am trying to solve linear elasticity for orthotropic materials with different orientations.

For the transformation of the coordinate system I take two perpendicular directions and the third axis should be the cross product of the other two.

Using the built-in cross() function in my variational formulation leads to an error in the assemble-process. When I simply replace the cross() function by a "by hand" calculation the code works. So using line 53 in the attached code works, using line 54 instead leads to:

In instant.recompile: The module did not compile with command 'make VERBOSE=1 ', see '/home/zorn/.instant/error/a529ee818965af71052c02fb07cb839c57e600ab/compile.log'
Traceback (most recent call last):
  File "wuerfel.py", line 98, in <module>
    A = assemble(a, exterior_facet_domains=boundaries)
  File "/usr/lib/python2.7/dist-packages/dolfin/fem/assembling.py", line 169, in assemble
    common_cell=common_cell)
  File "/usr/lib/python2.7/dist-packages/dolfin/fem/form.py", line 56, in __init__
    common_cell)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/jit.py", line 66, in mpi_jit
    return local_jit(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/jit.py", line 154, in jit
    return jit_compile(form, parameters=p, common_cell=common_cell)
  File "/usr/lib/python2.7/dist-packages/ffc/jitcompiler.py", line 77, in jit
    return jit_form(ufl_object, parameters, common_cell)
  File "/usr/lib/python2.7/dist-packages/ffc/jitcompiler.py", line 212, in jit_form
    cache_dir = cache_dir)
  File "/usr/lib/python2.7/dist-packages/ufc_utils/build.py", line 64, in build_ufc_module
    **kwargs)
  File "/usr/lib/python2.7/dist-packages/instant/build.py", line 541, in build_module
    recompile(modulename, module_path, new_compilation_checksum, build_system)
  File "/usr/lib/python2.7/dist-packages/instant/build.py", line 150, in recompile
    instant_error(msg % (cmd, compile_log_filename_dest))
  File "/usr/lib/python2.7/dist-packages/instant/output.py", line 49, in instant_error
    raise RuntimeError(text)
RuntimeError: In instant.recompile: The module did not compile with command 'make VERBOSE=1 ', see '/home/zorn/.instant/error/a529ee818965af71052c02fb07cb839c57e600ab/compile.log'

My system is ubuntu 12.04 64 bit and my fenics installation is the actual ppa-installation (1:1.2.0-1~ppa1~precise1).

Can anybody tell me if I found a bug or if my code is wrong?

Thanks in advance!