Hi Jessica,
Thank you for reposting this error over here on the Google Groups and for providing a screenshot of the console along with details of what steps you tried to remedy the issue. I am surprised even with a new spreadsheet that the error persists.
What about the rubric, did you make a copy of this rubric and try to attach the copy? ( make sure this file is on your drive, not team drives)
Are you using Doctopus with Google Classroom or is Doctopus creating and distributing the files for you?
The error might be pointing to that it can not access the rubric to use it or it can not access the student assignments. ( I am sure you can access the student documents yourself without an issue, correct? and these files are also on your drive)
Another possibility, have you recently deleted any odd files from your drive? Each time doctopus ingests an assignment it makes an association file, if these files are deleted, I will cause an error.
Go to the Trash on your Drive and see if any file types look like the ones in the image below, if yes restore them and see if that solves the issue.
Or you can try to reingest the assignment and then reattach the rubric.
Apologies, I know this is a lot of steps to look through, I am just trying to narrow down what is the cause is. Thanks!!