Reinstall Horde on Plesk, because of misbehaving (like authentication issue, broken images)

Please take a backup of the Horde database first to avoid any sort of data loss.

/usr/local/psa/admin/bin/webmailmng --remove --name=horde
rpm -e --nodeps psa-horde psa-imp psa-ingo psa-kronolith psa-mimp psa-mnemo psa-turba
mv /usr/share/psa-horde{,.$(date +%s)}
/usr/local/psa/admin/bin/autoinstaller --select-release-current --select-product-id plesk --install-component horde --reinstall-patch
/usr/local/psa/admin/bin/webmailmng --install --name=horde
/usr/local/psa/admin/bin/webmailmng --enable --name=horde
/usr/local/psa/admin/bin/httpdmng --reconfigure-all

If you see errors when rebuilding the Apache configuration via /usr/local/psa/admin/bin/httpdmng --reconfigure-all you should inspect any offending configuration items, address them, and then run the command again.