"Client prematurely closed connection" error

[cyoho]

I am using nginx version 1.16.1 as a forward proxy on a CentOS 7 server. 
Our order inventory server (ORDINV) connects through the nginx server
(TAXPROXY) to request sales tax info from a cloud server.   We are in test
mode with this configuration.  This all has been working well, except when
they stop testing for a while, the taxproxy server stops working.   The
error log shows 


2020/02/17  08:33:07  [info]  879#879:  42826 epoll_wait() reported that
client prematurely closed connection, so upstream connection is closed too
while SSL handshaking to upstream, client: xxx.xx.x.xx,  server:
taxproxy.umpublishing.org, request: "POST / HTTP/1.1", upstream:
"https://yy.yyy.yyy.yy:443/services/CalculateTax70", host:
"zzz.zzz.z.zz:80"

After a couple of these info there is [error] "peer closed connection in SSL
handshake (104:Connection reset by peer) while SSL handshaking to upstream"

The nginx service is running, and if I issue a systemctl restart nginx,
everything starts working again fine.   Any ideas what might be wrong?  
Googling turned up several sites where people reported similar problems but
no one had gotten an answer....

TIA~
Cindy

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,287065,287065#msg-287065