OK, so the team investigated this further. They're pretty sure you did an import that causes an issue in the API because of unexpected formatting. Specifically a document link.
There is a Group “Legal Briefs” defined in the custom type “LJC Case”. One of them is a document link (broken). The other links are attachment links.
To resolve this you need to go over all “LJC Case” documents and remove these broken document links.
The other option is to wait until we deploy the fix they made, which would be deployed this week or next.