The error message "connect() failed (111: Connection refused) while connecting to upstream" often indicates an issue in a server configuration or a service dependancy. For nginx, such errors typicaly involve commmunication with an upstream service, such as PHP-FPM for PHP processing.

When diagnosing this error, consider these primary causes:

1. PHP-FPM Service Not Active: Verify whether the PHP-FPM service is running. Use the following command to check if the process is listening on the expected port (9000 in the default configuraton):

   netstat -tuln | grep :9000
   

   If the service is inactive, start it by executing:

   /usr/local/php/sbin/php-fpm
   

2. Exceeding PHP-FPM Process Limits: PHP-FPM configurations limit the number of concurrent children processes that can handle requests. If this threshold is exceeded, incoming connections might be refused. To resolve:
   • Open the PHP-FPM configuration file php-fpm.conf and locate the directive:

     pm.max_children
     

   • Increase its value appropriately based on traffic and system resource availability.
   • Save the changes and restart PHP-FPM to apply:

     service php-fpm restart
     

By identifying whether the service is inactive or constrained by resource limits, you can address the "Connection refused" error and restore normal functionality.