Update: There was a mistake in naming the materials. With things corrected, however, the problem remains.
I attach the file (oxygen_diffusion.py). Running it results in
ValueError: data of variable are not set! (q, step 0)
When the material parameters are set to constants, the computation runs.
The same happens when using the config-file with `ts` time-stepper, e.g. changing  to time-dependent problem (time_field_dependent.py).