Yes the space was missing. But it has no positive effect.
Here the main part from the haproxy.cfg:
frontend public
bind *:80 v4v6
bind :::443 v4v6 ssl crt /etc/haproxy/certs/mydomain.pem
option forwardfor except 127.0.0.1
use_backend CR10 if { path_beg /CR10/ }
use_backend Ender7 if { path_beg /Ender7/ }
use_backend webcam1 if { path_beg /webcam1/ }
use_backend webcam2 if { path_beg /webcam2/ }
default_backend webcam1
backend Ender7
acl needs_scheme req.hdr_cnt(X-Scheme) eq 0
http-request replace-path ^([^\ :])\ /Ender7/(.) \1\ /\2
http-request add-header X-Scheme https if needs_scheme { ssl_fc }
http-request add-header X-Scheme http if needs_scheme !{ ssl_fc }
option forwardfor
server octoprint_1 127.0.0.1:5001
http-request add-header X-Script-Name:\ / Ender7
errorfile 503 /etc/haproxy/errors/503-no-octoprint.http
backend CR10
acl needs_scheme req.hdr_cnt(X-Scheme) eq 0
http-request replace-path ^([^\ :])\ /CR10/(.) \1\ /\2
http-request add-header X-Scheme https if needs_scheme { ssl_fc }
http-request add-header X-Scheme http if needs_scheme !{ ssl_fc }
option forwardfor
server octoprint_1 127.0.0.1:5002
http-request add-header X-Script-Name:\ / CR10
errorfile 503 /etc/haproxy/errors/503-no-octoprint.http
backend webcam1
acl needs_scheme req.hdr_cnt(X-Scheme) eq 0
http-request replace-path /webcam1/(.*) /\1
server webcam_1 127.0.0.1:8080
errorfile 503 /etc/haproxy/errors/503-no-webcam.http
backend webcam2
http-request replace-path /webcam2/(.*) /\1
server webcam_2 127.0.0.1:8081
errorfile 503 /etc/haproxy/errors/503-no-webcam.http