Online Guardian is a parental control software. It is also an anti-spam software that Forumotion uses to help safeguard its member forums.
Apparently it is not perfect(!), and can from time to time block people from entering their forums. If it let you in after entering your info in the form, then I think you should be fine. I can't be 100% sure, but it is not an uncommon problem.
There are a number(!!) of threads on it in the FM support forum:
http://help.forumotion.com/search?mode=searchbox&search_keywords=online+guardian&show_results=topics