i had this error while running zmbackup

$ zmbackup -f -a all
Error occurred: error while proxying request to target server: HTTP/1.1 503 Service Unavailable
all services seemed to be running

$ zmcontrol status
Host mail1.mydomain.com
	convertd                Running
	mailbox                 Running
	spell                   Running
	stats                   Running
this is the first mailbox of 2 mailbox servers. It also supposed to run proxy but even after i ran zimbra-proxy, it is not stated by zmcontrol status output

$ zmnginxctl status
nginx is running.