I have a test environment and am testing your related items xtd module.
The error that I get is:
Notice: Undefined variable: related in
<my server path>/httpdocs3/modules/mod_relateditems_xtd/helper.php on line 353
When I look into the code, I see:
It seems that no related articles are found at that point, so $related has no content.
It is easy to reproduce this error, just have one article with a keyword that is 'single', so no matching articles are available.
Edit: Naturally forgot about the Error Reporting setting. This is not a real problem. Please disregard.