"General socket error" when Nginx behind HAProxy using a unix socket
I had the same problem with Haproxy 1.8.19 on Debian Buster with Varnish 6.0.3 as backend.
For me, the problem come from chroot option in Haproxy configuration :
You have two solutions to fix this :
Move your's Nginx Unix socket in chroot directory
Or disable chroot option, but it's not recommended for security reason.