call_user_func_array() expects parameter 1 to be a valid callback, class 'Swift_Message' does not have a me thod 'addTextHeader' code example

Example: Warning: call_user_func_array() expects parameter 1 to be a valid callback

Somewhere in your theme or plugins is a line like this:

add_filter( 'something', 'regis_options' );
Could also be add_action(). Find that piece of code and remove or fix it.