When there are no tags for a post, and it is set to have related posts, it is breaking instead of posting a "No Related Posts" or similar message. I am coding a fix for this for a client, but you ought to update the plugin.
erp_display.php line 149 needs to have a conditional for if get_the_tags is not returning anything.
same with erp_widget.php line 180.
I don't know if those are the only errors of this type (for loop attempting to run on invalid variables) as these are the only ones that affected my specific client's setup.