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It seems you need update your Plesk up to 7.5.4 and install 2 hotfixes available from sw-soft.com
Then you can try to remove webmail from IIS and then reconfigure it with "websrvmng.exe --reconfigure-webmail" from %plesk_bin%
If this will not help then you can modify Plesk installation via add/remove programs and uninstall Horde. Then delete folder webmail from vhosts folder and reinstall Horde via add/remove programs
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)
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.