Since we've still got to investigate this a bit more, it sits here before posting as a bug ...
Our content guy ran into an error while attempting to create a new piece of content.
Interestingly, he created the content record, saved it, then went back in and added HTML content to the document.
When attempting to save the record he got a message like
No matter what he does, when it goes back to the content 'window' he gets a message about an unexpected iMIS error. Changing the Content Layout from (None) to something else (like SingleColumn) allows him to add the html content successfully.
I've only found two tables that seem to help with how content is stored, and none of them have this contentItemKey value (and a search on iMIS Community doesn't turn up any results).
Adding content to the record before initially saving it works fine.
15.1.1.2998
Two semi-relevant tables
DocumentMain and Tag are the two tables that I've thus far found to be important.
Unfortunately, I've been unable to determine where, for example, the actual HTML content is stored in SQL (I assume it's still stored in the db somewhere).