Remove blank space from below iframe on page?

Here is the website with iframe in question.

I am using iframe resizer by davidjbradshaw and when you first load the forums tab up, the iframe gets loaded but it creates an empty space between it and the bottom of the page. It won’t go away until you scroll all the way down, and then scroll back up. It doesn’t do anything like that on the dynmap section. I’ve also disabled the script to see if that was causing it to create blank space but it’s not. It seems to be a div on the bottom of the html doc that creates a random size, but I can’t figure out what it’s from.