this form contains a hidden field that does not have a valid default value configured

[David Sopkin]

How do I find the hidden field that is blocking ticket creation?

[Peter Bengov]

Hi David, are you using Server? If yes - you can just review the logs. 

Otherwise please share a bit more data about the form you are talking about.

Overall, you can go by elimination field by field setting and changing the values or mandatory settings.