[horde] Horde login issue

[Shamus]

I have kind of a weird set-up, in that I have two domains, one that
physically hosts my website- call it foobardomain.com- and one that
redirects web users to foobardomain.com- call it fbdomain.com. I want
to use fbdomain.com for email. (Rather, the site owners want that, as
it's easier to type when they go to check their email.)

I've set up duplicate email accounts on both domains, with the ones on
foobardomain.com forwarding their mail to their counterparts on
fbdomain.com. The problem is, I can't log in to Horde on fbdomain.com,
but I can on foobardomain.com.

To complicate matters further, this was all done within PLESK 8.6.

I need advice on what might be causing this. TIA.