One of our site owners encountered an error when trying to create a Nintex form on a new list. The error message was as follows:
The key part of the error outlined in red was, "The server encountered an error processing the request. The exception message is 'Index was outside the bounds of the array.'"
I could recreate the issue on that list with my admin account, but not on other lists on the same site. I didn't find any community discussions about this, so I started looking at the list structure and settings. I quickly noticed an unusual column. One of the columns was named "#". I changed the name to "Number" and retried launching the forms builder. Lucky guess! It worked! To test it out, I tried some other special characters and ran into the same error. Interestingly, if the column name had text and a special character, it seemed to work fine, but when it was just a special character, I received the error. So, for example, using "#" or "$" as the column name resulted in an error. Using "# of users" or "$ amount saved" did not result in an error.
The lesson... avoid using just special characters as your column names.
For reference, we're using SharePoint 2013 on premises, and Nintex Forms version 220.127.116.11
Hope this helps!