Error 400 Unexpected error from sharepoint

Help!
1

We have an interesting problem.

When we use a link to a site that goes through our HAProxy it gives us an error 400 unexpected error. If we then go to the adressbar of the browser and press enter without changing the adress it just works.

The link is on a sharepoint and is very simple. Just this:

<a href="https://cmg.domain/BluStarWeb/Start" target="_blank">text</a>

Our configuration in HAProxy is this:
frontend https_front

    bind *:443 ssl crt /etc/haproxy/certs/domain.pem
    reqadd X-Forwarded-Proto:\ https

    acl host_cmg hdr(host) -i cmg.domain
    http-request redirect scheme https code 301 if !{ ssl_fc }
    use_backend bak_cmg if host_cmg

    backend bak_cmg
    server  petelcmg        192.168.2.87:80 check`

Any ideas what could be the problem?