Background... we have a lot of students with one name (not a first name and last name) and the institution has got around this by using '*' as the first name. 

To reproduce

This requires the Windows operating system. I tested on Windows 10

1. Create a new user. Make the first name just '*' and the last name anything you like. 
2. Create another user with a firstname and lastname just for comparison
3. Create a course and create an Assignment within that course. Allow file uploads
4. Enrol both students
5. Edit both users submissions and add a test file
6. Click 'Download all submissions' next to 'Grading actions'
7. Open the resulting zip file in the standard Windows Zip program

The folder for the '*' user is not there as it would not be a valid Windows pathname (see https://docs.microsoft.com/en-gb/windows/win32/fileio/naming-a-file)

By comparison, it works fine in MacOS and Linux. 

Before somebody says "don't use * as a first name", my point is that it is possible to create a valid user account with default settings that results in an invalid zip file in Assignment.