Seafile/Mattermost/Collabora cannot be accessed in Plesk: is enabled but not reachable for this domain

Symptoms

• Plesk Premium Email (Kolab) Troubleshooting tab in Plesk UI shows:

  Seafile is enabled but not reachable for this domain;
  Mattermost is enabled but not reachable for this domain;
  Collabora is enabled but not reachable for this domain;

  

• Address Book in webmail is empty, it does not show any contacts; 

• Accessing Seafile in Extensions > My Extensions > Plesk Premium Email - Powered by Kolab shows error:

  Authentication failure for Seafile Storage

• Accessing Mattermost in Extensions > My Extensions > Plesk Premium Email - Powered by Kolab menu shows:

  404 not found

Cause

Nginx is not installed or is not enabled on the server:

In Tools & Settings > Services Management Reverse Proxy Server (nginx) is marked as:

• Service is not installed
  or 
• Service is not running

Resolution

1. Log into Plesk.
2. Install or enable Nginx:

   In case Reverse Proxy Server (nginx) is marked as Service is not installed

   install it in Tools & Settings > Updates >Add/remove components page (Web hosting >Nginx web server and reverse proxy server menu)

   In case Reverse Proxy Server (nginx) is marked as Service is not running

   enable it by clicking Start the Service: