Hello @Gunther_Joritz,
I have updated your course and header. Kindly be careful from next time when you select the specialisation and course while posting a new topic. There are some instructions given when you create a topic, follow those instructions.
Now comes to your error, most likely there is something damaged or edited in your notebook that the grader depends on.
First get a clean copy of the notebook (see the relevant topic on the FAQ Thread)
Then re do your assignment by referring to your saved copy of assignment and then submit again.
Let us know if your issue gets resolved.
Regards
DP