I have several 2860's setup and working, and have typically used https only to access from WAN, changing the default port if required.

One I have running on a remote site which I setup last week, and while on site, I checked I could see the login page remotely, which I could.

Now I am not on site, if I go to login, I get the login page, I enter the user and password, but it just pings back to the login screen (NO "incorrect password" type error in red text). If I put in the wrong password or user, it warns me with red text. If I use the correct one again, no warning in red, just the login screen. With this in mind, I know I am using the correct details.

What could be causing this issue?

You haven't got SSL VPN enabled have you?