I'm trying to create a Nintex form in Sharepoint 2013 that helps prohibit double-booking in a Calendar List. To do this I've created a relatively simple formula, using a rule of type "Validation". The formula is
When using trying to submit the form I get an on-screen error message that states "Validation Error - There was an issue validating the data entered on the form; please try again. If the problem persists, contact the form owner.". In the browser's debugger I see "Handling all asynchronous validation rules failed". I've been able to validate that the proper data comes back from the server using Fiddler to inspect my web traffic and seeing the JSON response that contains the event that otherwise fulfills the lookup query.
For an interesting twist, I used the same formula for a "Formatting" rule, and that fires just fine turning the label red to help indicate the problem.
Is there a better way of preventing double-booking of an event in a Nintex form, or perhaps something I'm not doing right here?