After a Plesk upgrade, it’s possible that some services don’t do what they’re supposed to be doing (and have done right, before the update). It could be a Horde Webmail that is displaying a default Plesk page, instead of the webmail-client, or it could be something entirely different.
One of the easiest things to try, and one that solves a lot of Plesk issues, is running the command that will re-configure all services on the server, according to the Plesk internal database. That database contains all settings for mailboxes, DNS-records, web-users, additional settings, …
Here’s how to run it
/usr/local/psa/admin/bin/websrvmng -av
It’s harmless – as your services installed should always be configured with the latest Plesk Database Information (this normally happens automatically).
Note for Plesk 10 and later: websrvmng -av is obsolete and will just print a warning telling you to use httpdmng instead. The current equivalent that rebuilds all web server configs from the Plesk database is:
/usr/local/psa/admin/sbin/httpdmng –reconfigure-all
If that doesn’t do the trick to get Horde Webmail back, your best bet is the official Plesk support knowledge base, since the old SoftLayer KnowledgeLayer article this post originally linked to no longer exists.