I would suggest you email one of the admins there and ask. Explain the situation and see what they have to say. It could be that someone tried to register using your email a while back. It could also be that someone was banned from the site and they banned the whole IP range, which yours may fall into.
You won't know unless you contact one of them. No offense, but asking here won't do anything but have someone (other than you) start a "board war" which Eric has stated many times he does not want.
FWIW, I signed up a while back using my yahoo account. I have had no problems with it.