I came across an error which is not common with the latest versions of Plesk. However, Horde mail client was showing errors while trying to add an attachment to the email while composing it.
unable to create VFS directory /.horde/imp
In order to fix you, please check the horde user details in /etc/passwd first.
[[email protected] ~]# grep "horde" /etc/passwd
The result will be similar to the line below.
horde_sysuser:x:123:45678:horde webmail user:/usr/share/psa-horde:/sbin/nologin
Then, please check the ownership of the directory .horde in the /tmp. Reset it to the ownership horde_sysuser:horde_sysgroup if its different.
[[email protected] ~]# chown -R horde_sysuser:horde_sysgroup /tmp/.horde/
This will fix the issue with attachments of the Horde email client in Plesk.