First, you have a typo in your two horizontal files. The line below has an extra ">" character. Simple fix.
<input id="sclogin-remember" type="hidden" name="remember" class="inputbox" value="yes"> title="Remember Me" />
But beyond that - when I use the modal login, I have some really weird formatting issues, including the close "X" being a few pixels off in its hot zone and clicking outside the modal, which should close it, not working.
My first guess is bootstrap, but that's just a guess. I'm using a RocketTheme template, which brings in Bootstrap. Any chance it's using a version your code doesn't like?
Take a look - the site is in development now, so nothing can be broken.
www.nerdsmart.com ... note the login link on the right side roll. Clicking it brings up the rather poorly-formatted modal.
Any chance I could see the code and CSS for YOUR modal login screen? I'd love to copy it as a starting point. Regardless, I'm unsure where to start trying to debug this.