Hi @ambrosite, thanks for the plugin and extensive support you give.
in_same_tax looks like it's specifically designed not to work with non-hierarchical taxonomies e.g. tags.
I guess that's intentional and there's a good reason for it.
I needed it to make it work so removed the array_filter from line 901 (to give $taxonomies = get_post_taxonomies($post->ID);)
I'm posting this:
(a) to say that it would have been useful if in_same_tax did support flat taxonomies
(b) to help anyone else with the same issue
(c) to ask you if this is going to break things in ways I'm not foreseeing.