Parent / Child Fields Suddenly Not Working


I have been using the Parent / Child Drop Down field type for almost a year. Suddenly they don't work. I can create the column, but when I try to open a new list item, I get either an "Unknown Error Occurred" or "Field was not defined properly" errors. No errors in the event logs on any of the servers, and nothing has changed on any of the servers that I know. Anyone have any ideas? Microsoft's suggested fix was to remove the solution and try to reinstall it.