The following message appears when opening the Inbox using Horde:
CONFIG_TEXT: IMAP error reported by the server.
The server is using qmail and courier was recently updated.
# grep -i courier /var/log/yum.log
This is a Plesk bug with ID #PPPM-11161, which has been already fixed on Plesk Obsidian 18.0.21.
In order to fix it, upgrade to Plesk Obsidian following the instruction found here
If the upgrade is not possible, as a workaround, use Roundcube until the issue is resolved.
The steps to install and switch to Roundcube can be found here: How to install and manage webmail in Plesk?