This answer says it occurs when it sends twice.
Mine activates when a user changes the value in a selection box, so this isn't my problem.
Here is my previous question which shows my code and a few screenshots of the errors.
I couldn't get the 500 (Internal server error) working, hopefully solving the ajax problem first will help sort it out.
The full error from the console:
error admin-add-module:128
$.ajax.error admin-add-module:128
l jquery-1.8.3.min.js:2
c.fireWith jquery-1.8.3.min.js:2
T jquery-1.8.3.min.js:2
r jquery-1.8.3.min.js:2
After changing JQuery to the full version it output this error: