System Error when submitting a document to a specific category

[Richard H.]

This is very odd. Submitting a document to a particular category (19.1 Staff Site) in our library causes a 'System Error'. Submitting the same document to another category (19.2 Policies) works fine without any issues.

What has gone wrong in category 19.1? We can't see anything obvious. There are other documents within that category that are appearing on the web site as expected (with some of those documents owned by the same author as the document it didn't like).