Hello Dory,
Are these files (assignment folder and rubric) on your drive (not on a team or shared drive)?
Next time this happens can you please hold down Control+Shift+J" on a PC or "Command+Option+I" on a Mac and when the side window opens, take a screenshot of the error console ( make sure the side window is on the 'console' tab). I would be curious to see if the error console can help us pinpoint the issue you are seeing.
Thanks!