There were a couple a white spaces in Drop Down spaces in our unbound fields.

We removed the white spaces for all the drop down lists within Nintex form and it WORKED!

I am also encountering this error. Can you please clarify the white spaces issue? Are the white spaces in the values or the description causing the issues? Obviously having a white space in a Drop Down item is ideal.

Josh Lukins‌, did you ever figure this out? Where are these white spaces?

Nope - We have still been experiencing this error. I did not pursue the White Space theory as I didn't get clarification on where to change them.

In my circumstance, the error seems to happen at random about 10% of the time. This makes it very difficult to work out what is causing the problem.

Well, I did end up having some success, but it's still not ideal.

I'll post my code and the entire solution once I have it finished, but in my situation, I'm using JavaScript to query three different lists across two different sites. I use data from two of the lists to filter data from the third list before building my drop down control.

From all the research I did, it seems like the postback doesn't like 'new' entries added to the drop down. So, I decided to use a List Lookup control to query the master list and return all items. Then, I ran the rest of my code to re-query the list using my added filters. The code then blows away all of the results in the drop down and rebuilds them in the identical format, but with the limited results. 

That seems to be working right now. Apparently, you can have a drop down with fewer options than what the page initially loaded, but add something that wasn't there to begin with and it'll blow up.