Your comments

Could you please try using IPv6 again with the latest Ajenti release? It seems to work well now for me (either with direct [::1] hostname or IPv6 DNS entry).
Thank you for the info! I'll make these changes asap and notify you.
Arthur,

Is there a real difference between two approaches? Doesn't PHP_VALUE work?
You can also see nginx status and config check result in  Configuration Checks under Websites
Did that fix it?
Looks like you have a reverse proxy entry in ajenti v, which doesn't have URL set.