NC Docker, PicoCMS, HAProxy - no luck so far

I hve set up a PicoCMS Mini Website via a NC20 (Docker Container).
Both NC and PicoCMS work fine.

I am trying to rewrite another Domain to this PicoCMS website,
so basically i want to point to

My setup works partly, but the assets (.js/.css) are not served, just the pure text.
I have tried several things, regexp rewrites etc. without luck.

does anyone have a similar setup and can give advice what i might be doing wrong?

frontend https
bind ssl crt /etc/haproxy/cert.pem
mode http
log global
option log-separate-errors
option httplog
option http-keep-alive
option forwardfor
acl https ssl_fc
http-request set-header X-Forwarded-Proto http if !https
http-request set-header X-Forwarded-Proto https if https
timeout client 30000
acl nextcloud var(txn.txnhost) -m str -i
acl picocms var(txn.txnhost) -m str -i
http-request set-path /index.php/apps/cms_pico/pico/website/ if picocms
use_backend nextcloud if nextcloud
use_backend nextcloud if picocms

backend nextcloud
mode http
id 40
log global
timeout connect 5000
timeout server 86400000
retries 3
server nextcloud id 139