Haproxy acl - service unavailable

I'm trying to setup haproxy acl, and it gives me 503: Service unavailable error, even redirect by port works fine. What am I doing wrong? Appreciate any help.

This doesn't work by x.x.x.x/havana :

   frontend https
            bind *:80
            mode http
            option httpclose
            acl otter-path path -i /havana/
            use_backend otter-server if otter-path

This shows backend fine by x.x.x.x:82 :

 frontend otter-server
            bind *:82
            option forwardfor
            default_backend otter-server

Backend configuration:

backend otter-server
        server otter2 192.168.0.15:8004

Solution

the problem was - it redirects not to backend, but to backend/havana, which doesn't exist.

Solution is to remove subpath after redirect, so it points exact to backend root

backend annotrack-mouse
        balance roundrobin
        http-request set-uri %[url,regsub(^/havana/mouse,/,)] if { path_beg /ha$
        server annotrack-mouse 192.168.0.10:3000
        option httpchk