Unable to login to Plesk UI: a blank screen is shown

Symptoms

• Plesk login page shows a blank screen.

• 
.html
links like https://plesk.example.com:8443/example.html are successfully loaded.

• Websites are accessible.

Cause

Master sw-engine process is hung.

Resolution

1. Connect to the server using SSH.

2. Kill all  sw-engine processes:

   # pkill -f sw-engine-*

3. Run the the following command to show if there are any leftover process that were not killed in previous step, for example:

   # ps auxww | grep sw-engine
   psaadm 7478 0.1 0.6 322832 6576 ? S 05:41 0:00 sw-engine-kv
   root 7479 0.0 0.9 482148 10192 ? Ss 05:41 0:00 sw-engine-fpm: master process (/etc/sw-engine/sw-engine-fpm.conf)

4. Kill the processes listed above:

   # kill -9 7478 7479

5. Start the sw-engine service:

   # service sw-engine start

6. Start the "Web-Socket" service as well:

   # service plesk-web-socket restart