Loadbalancer sporadic 503 with round-robin DNS

Hello, we are using Rancher on 3 hosts, IPs to
We’ve setup a lb stack with haproxy running on every host, port bindings


Our DNS is serving these IPs round-robin for sq.mycompany.com, so an arbitrary host is adressed on each request.
The balancer rules forward public ports 80 and 443 to mystack/sonarqube:3000.

Now the problem: Sometimes this works, sometimes we get 503 when accessing sq.mycompany.com. Using a different browser often solves the problem.

At this point I’m really stuck, can anyone give me a hint what to check or to fix?