Hey Matth,
I’m sorry but I could not replicate that notice therefore I could not guarantee I could point you to the right direction.
Here’s line 1725 from item-skin.class.php:
$lb_post_title = $is_post ? $base->getVar($this->post, 'post_title', '') : $this->get_custom_element_value('title', $separator, '');
The notice states that there’s something wrong with the separator taken from the Layer Settings of the skin. In case you’re not familiar with the Skin Editor, please see https://www.themepunch.com/essgrid-doc/item-skin-editor/
$separator = $base->getVar($layer['settings'], 'source-separate', ',');

Please check what you have placed in there. If it’s empty, please try inputting something like a space.
If that does not help, we’ll need to post this in our issue tracker for review.
Thanks.