Grading Failed - Grader Error: Malformed feedback

You need to “undo” the changes in question. If you are in a situation in which you’ve lost the information about how it originally was, you can start over with a clean copy of the notebook and then “copy/paste” over your legitimate changes. Here’s a thread from DLS that explains how to do that. The method is not specific to DLS.

Please give that a try and let us know.