The problem you have reported is affecting a few courses and we are informed that Coursera is looking into this matter. Before they come up with a fix, one of our course staff has suggested a temporary workaround in this post that will change the permission of an assignment notebook. Would you mind giving it a try and let us know whether it works for you?
Well, I chatted with one of the support staff this afternoon and he/she said that: “I searched about this error you reached and we do not have records of this being an issue with the Course and assignment”
I guess the poor guy was not aware of it, hope they’ll fix it quickly though
EDIT: again, thanks for the fast answer, that’s really cool