function related_tag_posts and function related_search_tag_posts have a leftover "wp_posts.ID" as part of the query, so since my site has different table prefixes plugin was returning nothing.
Even after fixing the query, I am not convinced about the results.
Using only "posts related to the shown post" and "posts related to seach engine terms (tags and categories)".
Two test posts that are in the same category and share one tag.
Clicking on post "a" shows post "b" in the widget, as expected. Clicking on post "b", however, does not show post "a". Other posts, not in the same category, but with one tag in common with post "b" are shown.
Are you factoring in categories? In my usage categories carry much more weight than tags and I would expect two posts in the same category with a matching tag to be considered related.