Hello @yuluma - try going to Server > Services and then select the ‘Heartbeat’ option in the overflow menu. The output may clarify which site is having the issue. If it’s not clear, another thing to try is SSH’ing into the server and then run nginx -t which may provide the needed details.
root@webshops1:~# nginx -t
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful
You might try cross referencing the client IP to the NGINX access log to see if that helps identify a site. It may also just be a client that had some issue loading SSL.