Why does HAProxy show that a server's check URL is a 404 when running curl on this URL is successful? - haproxy

Question

I'm setting up HAProxy to load-balance a resource between 3 back-ends. Here is the HAProxy config : (In the following snippets I replaced the actual domain name by example.net)
global
log 127.0.0.1 local2
log-send-hostname
maxconn 2000
pidfile /var/run/haproxy.pid
stats socket /var/run/haproxy.sock mode 600 level admin
stats timeout 30s
daemon
# SSL ciphers
...
defaults
mode http
option forwardfor
option contstats
option http-server-close
option log-health-checks
option redispatch
timeout connect 5000
timeout client 10000
timeout server 10000
...
frontend front
bind *:443 ssl crt /usr/local/etc/haproxy/front.pem
reqadd X-Forwarded-Proto:\ https if { ssl_fc }
stats uri /haproxy?stats
option httpclose
option forwardfor
default_backend back
balance source
backend back
balance roundrobin
option httpchk GET /healthcheck HTTP/1.0
server server1 xxx.xxx.xxx.xxx:80 check inter 5s fall 2 rise 1
server server2 yyy.yyy.yyy.yyy:8003 check backup
server mysite example.net:80 check backup
The issue is the following: even though the first 2 servers respond correctly, the domain-based one always shows as a 404:
What is counter-intuitive to me is that if I use curl to access this same healthcheck, I get an HTTP 200 (like I would expect to see in the HAProxy stats) :
curl -I http://example.net/healthcheck
HTTP/1.1 200 OK
When I ping my site, I get:
# ping example.net
PING example.net (217.160.0.195) 56(84) bytes of data.
64 bytes from 217-160-0-195.elastic-ssl.ui-r.com (217.160.0.195): icmp_seq=1 ttl=50 time=45.7 ms
Is it because the IP of my domain is shared with other domains (1&1 shared hosting) that HAProxy can't access it? Why is that and how to make HAProxy reach it correctly?