After migrating Plesk domains to another server and fixed couple of SSL and domain issues I’ve noticed webmail subdomains are showing Plesk’s Domain Default Page. Everything was fine, no installation problems. I’ve disabled Horde webmail.
Weird thing was new domains were working fine. I’ve did a research but no simple fix. Then I’ve checked Mail Settings of a subscription and noticed Webmail option was selected as None… So I’ve switched it to MailEnable WebMail and webmail problem was fixed. Don’t know a fast switch for all subscriptions. But needed to update this for all. Note that all my plans were switched and synced to MailEnable WebMail 9.00 by default.