Eureka, i found it…
I needed to add the following in my backend:
acl p_root path -i /
http-request set-path /xwiki if p_root
so my complete wiki backend now looks like:
backend bk_wiki
option httpclose
option forwardfor
http-request set-header X-Forwarded-Port %[dst_port]
http-request add-header X-Forwarded-Proto https if { ssl_fc }
http-check expect string 200\ OK
acl p_root path -i /
http-request set-path /xwiki if p_root
server wiki1 10.1.0.22:443 ssl verify none maxconn 1000 weight 10 check