Plesk 12.5.30 suddenly won't show login screen

Last week I got this message when I tried to login at Plesk admin panel at https://<domainname>:8443

"502 Bad Gateway"
nginx

As far as I'm aware of, the server was not recently updated. (I checked the yum update log)

I searched for help articles and found this one and followed the instructions:

https://support.plesk.com/hc/en-us/articles/213955225--HUB-Troubleshooting-502-Bad-Gateway-error

tail /var/log/sw-cp-server/error_log

output: (ip and domainname masked)

<ip>,...

Plesk 12.5.30 suddenly won't show login screen