Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Plesk Uservoice will be deprecated by October. Moving forward, all product feature requests and improvement suggestions will be managed through our new platform Plesk Productboard.
To continue sharing your ideas and feedback, please visit features.plesk.com
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.