ur upstream is refusing the connection as described in the error log.
have you checked ur upstream logs? could be multiple things, starting
from server configurations to webserver config. check to make sure ur
not hitting ur max connection on the upstream httpd, paste an output
of ‘dmesg’, if ur running iptables or any other firewall makes sure to
check the logs and rules and check the system to confirm that u are
not running out of sockets to be used by the upstream httpd as well as
system file descriptors “ulimit -a”
if u want you can paste an output and i can help troubleshoot
cheers