WARNING: pool www seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers)

The error becomes when there is a request coming in and PHP-FPM needs to start a new child process to service a request, instead of using an existing one.

This happens because you have really small numbers set in your pool configuration.

The numbers mean that you are allowing a maximum of 10 simultaneous PHP processes running. You are starting with 2 children, and have only one minumum server ready to serve requests at any point. If there is some idle time on your web site, then PHP-FPM closes idle processes so that maximum three processes are online ready to serve requests.

I would multiply pm.max_children, pm.start_servers, pm.min_spare_serversandps.max_spare_servers` numbers each by 5, which would be a suitable starting number.

Then you can just monitor your traffic and check if these errors appear again, then adjust numbers accordingly.