Unable to login to Horde or browse any pages in Horde:
IMP is marked as authenticated, but no credentials can be found in the session.
The only mysql database server on 3306 port located in Tools and Settings > Database Servers is Remote (not localhost) and is in Not Connected status
Horde unable to connect to it's database
1) Fix the issue with the not connected mysql database server on 3306 port, or add localhost database server through Plesk Installer and mark it as Default.
2) Re-install Hord using instructions here.