After upgrading Plesk 9.x to Plesk 10.0.0 / Plesk 10.0.1 webmail is not working anymore.
When entering http://webmail.domain.tld the default Plesk page is showing.

Here is a simple workaround, please note that you need root access on your server.

Run the Plesk installation script in repair mode, replace pp10.9.0-bootstrapper with your own version.
This will take a couple of minutes depending on the hardware of your server.

# /usr/local/psa/bootstrapper/pp10.9.0-bootstrapper/

Once the installation has been finished we have to install, enable and configure the webmail.

# /usr/local/psa/admin/bin/webmailmng –install –name=horde
# /usr/local/psa/admin/bin/webmailmng –enable –name=horde
# /usr/local/psa/admin/sbin/httpdmng –reconfigure-all

That’s all! Webmail should now be accessible.