On my Debian Plesk Server I am hosting a domain and two sub domains. Everything was working perfectly for the last months without any code changes. Today I noticed that every few requests (klick of a button, navigation to another page,…) error 503 appeared and disappeared with another refresh.
I tested it with two simple php files that are connected with an tag. The result was that every 3rd to 5th klick error 503 appeared. However, the same test works perfectly without any errors on the main domain.
Just to be sure, I created another subdomain and tested the two linked php files and it showed error 503 like the other subdomains after some requests.
Is it possible that the last Plesk update trashed the php config of the sub domains? Is there any chance to fix that issue?