Mystery gap at the bottom of certain pages


There's a mystery gap (see attached image) at the bottom of certain pages, and I can't figure out how to get rid of it.
These are the affected page:

I'm using a plugin called "Simplr User Registration Form Plus" and its recaptcha feature on these pages, so that could be the cause.

I'd really appreciate if you could help me get rid of this gap.
Thank you so much in advance