When I insert objects into a list in an object field of a document, I get error message:

Database error

Write failed with error code 17280 and error message 'WiredTigerIndex :: insert: key too large to index

What stands out is that the error only occurs with fields named 'aac'. Other object fields (in the images, the fields 'test', 'filters', 'aacc') allow insertion. I'm inserting dictionaries into lists that are in the object field.
If you rename one of the fields to 'aac', the error occurs.