call_user_func_array() expects parameter 1 to be a valid callback warning

[Valentina R]

The warning occurs in Appointment Calendar as seen in the screenshot.

Reported here: https://bizzthemes.com/forums/topic/call_user_func_array-expects-parameter-1-to-be-a-valid-callback-error/

[Željan Topić]

Hook customer used isn't valid any more. Add-on rearrangement is not available as add-on option.