Confirming error reported by houseofstrauss, using 2.8.5
Using default theme, and have 5 tags, 3 of which have posts. Did nothing to Widget except to drag it to sidebar.
Please note I flagged the compatibility as “Broken” for 2.8.5.
Also note, deactivating this plugin causes error:
Warning: Cannot modify header information – headers already sent by (output started at C:\xampp\htdocs\wordpress\wp-content\plugins\related-tag-filter\related-tag-filter.php:286) in C:\xampp\htdocs\wordpress\wp-includes\pluggable.php on line 865