WordPress call_user_func_array() expects parameter 1 to be a valid callback, class

This is a problem with the Better AMP plugin. Best to do is report it to the plugin developer.

There is an invalid filter hook in the plugin, for a quick fix you can try to modify the file wp-content/plugins/better-amp/better-amp.php.

Change line 1197 and 1198:

// From:
    add_filter( '...', array( $this, '_return_false' ) );

// To:
    add_filter( '...', '__return_false' );