Google classroom assignment comments not seen while turned in

In Google classroom you can make assignments.

In Assignments you can make a google doc for each student.

Students can turn in their assignment.

While you're marking the assignment, you can make comments.

Students CANNOT see the comments until you return the assignment.

BUT they can see any text you type in the document itself.

They and you can see when the other person has the document open.

Mystery: why Google classroom sometimes claims you don't have permission to share a file in About Materials

Google classroom has an "About" section that you can post administrative materials in.

Typically it works just fine, but today I saw on a Mac that with both a Word doc, as well as a Google doc, it gave the following error message when we tried to post it:

Oops! There was a problem attaching a file. Make sure you have permissions to share this file and try again.

There's a Google group forum where people are complaining about this, but so far no solutions. 

By the way, we did try to clear the cache, and no luck, as others who posted after that suggestion also found.

I have not bee able to reproduce this on my Mac.