Those who are having issues with "503 Service Unavailable" only for internal access might want to try this:
Part 7 Step 4:
Services --> HAProxy --> Settings --> Rules & Checks --> Conditions
Don't set the condition to "Source IP is local" but select "Source IP matches specified IP" and input the private IP ranges you actually need, e.g. 192.168.0.0/16 10.0.0.0/8 172.16.0.0/12.
This fixed the issue for me.
Also, in case your local servers only accept http connections, make sure to remove the SSL checkmark of the respective "real server" entry. Even though that should be somewhat self-evident :-)
Part 7 Step 4:
Services --> HAProxy --> Settings --> Rules & Checks --> Conditions
Don't set the condition to "Source IP is local" but select "Source IP matches specified IP" and input the private IP ranges you actually need, e.g. 192.168.0.0/16 10.0.0.0/8 172.16.0.0/12.
This fixed the issue for me.
Also, in case your local servers only accept http connections, make sure to remove the SSL checkmark of the respective "real server" entry. Even though that should be somewhat self-evident :-)