It's impossible to say without more information than that.
What browser, what operating system, what specific steps are used to sign in, does it always happen, sometimes happen, happen under specific conditions, do you run anti-spam software, anti-scripting software, run flash blocking software, etc.
No one else is reporting this, so it's a pretty sure bet it is related to your system or procedure. If you could spell out what those are, maybe there is a clue there.