All users, including moderators and admins are able to send and receive PMs, but not to post new posts. (You can edit your previously existing posts.) Upon submitting a new post, the user gets the following error:
This problem just started last night, for no obvious reason, and it is happening to ALL users, whether or not they are running any kind of firewall. You can write a post, and you can preview the post, but you get the error when you submit the post.

This has been going on for about 20 hours now, and our board is dying. We have a support ticket in with Groupee, but I need this fixed NOW. Does anybody have any ideas?

Thanks.

You should really post UBB related erors at UBBCentral.

As for the error, it sounds like the host checker is in error; be sure your domain exists in the valid hosts listing (both the www and non-www version of your domain(s))
CP > Primary Settings > General Tab > Expand Advanced Options > Domains for HTTP Referre Check

You can also choose to disable the check from this page, though it's not recommended.