In some cases Horde mail client in cPanel will throw an error as User not allowed message. For the first try we can use one of the cPanel’s binary itself as shown below.
root@server[~]# /usr/local/cpanel/bin/update-horde --force
Then it shows the following error
!!! Horde Groupware Webmail Edition detected !!!
Install or update of standard Horde Webmail is disabled.
Horde Groupware Webmail was installed with cPanel 11.25.1.
Downgrades of Horde are not permitted due to MySQL differences.
If the issue still persists, you can go for the cPanel script to fix the issue.
Try running /scripts/fullhordereset in the server.
Now try to relogin to the e-mail account using webmail and select Horde as mail client, this time there will not be any issues with Horde.