Problem:

I am writing a custom filter to work with contactform7 that displays a selection field for the dates on which a certain presentation is given. It works fine on the front end.

However, when I try to access this particular form on the back end WordPress is reporting a critical error.

Solution:

The client fixed it by solution of another thread:

https://toolset.com/forums/topic/problem-with-the-latest-plugin-updates/

Relevant Documentation: