Thanks a lot @tibor and @davecahill, together you solved my problem using the Plainbox theme (based on K2) in the posts above. To me this is what did it:
To fix the error:
navigate to your K2 folder -> app -> includes
edit the file widgets-removal.php as below
Change the contents of the file from:
<?php
if (get_option(‘k2sidebarmanager’) == ‘1’) {
remove_action(‘plugins_loaded’, ‘wp_maybe_load_widgets’, 0);
}
?>
to
<?php
if (get_option(‘k2sidebarmanager’) == ‘1’) {
remove_action(‘plugins_loaded’, ‘wp_maybe_load_widgets’, 0);
if( is_admin() )
{global $pagenow; if( $pagenow == ‘index.php’ ) wp_maybe_load_widgets();
}
}
?>
(That was Tibor’s advice, with davecahill’s correction).