Websocket config problem

i have a HAPROXY frontend which terminates ssl and at the backend site, there is a http service which
answers on a subdir /socket with a websocket

I cannot get this to work
always the GET request is provided with wss:// of course the backend runs with http so it isn’t working

i get a 400 Bad websocket request

i’ve tried the usual stuff like:

acl is_websocket hdr(Upgrade) -i WebSocket
acl is_websocket hdr_beg(Host) -i ws

but the get request fails always with wss://

what do i need to do to make websockets work on the http backend?