This is a late response but I have had exactly the same problem and here is my solution.
webmail.domain.com, gave me exactly the same error.
when I tried webmail.domain.com/imp , I have found out that everything works fine.
Then I changed the "home directory" of horde to "C:\Inetpub\vhosts\webmail\horde
\IMP" (may differ in your setup)
(Start / All Programs / Administrative Tools / IIS Manager / Your Server Name / WebSites / Webmail(horde) right click / Properties / Home Directory / Local Path)
Then webmail.domain.com started to give errors like mentioned in the following posts:
http://forum.parallels.com/showthread.php?t=101624
http://forum.parallels.com/showthread.php?t=77979
After that, I changed the "Local Path" back to the original address. ("C:\Inetpub\vhosts\webmail\horde")
Then everything was back to normal and working perfectly!
Seems like it is kinda simple confliction in registery or something like that.
Hope works for someone else too.