[imp] IMP redirects to https on port 80 !?!?

[Gonzalo Servat]

Hi All,

On every second login attempt to IMP, it seems to be trying to redirect to 
https on port 80. So.. I type my login/password, click login and IMP tries 
to go off to:

https://www.domain.com:80/horde/imp/mailbox[...]

(notice the "https" and port "80")

Every other time, it works fine (ie. it goes to the right URL 
(http://www.domain.com/horde/imp/mailbox[...]).

What could possibly be causing this?!?

Thanks in advance for any help.

Regards,
Gonzalo