Help with docker compose and nginx reverse proxy on another domain

[astrakid]

hi,
i have opensign up and running on my local server.
now i want to have anotther reverse proxy (nginx) in font of it. but i am neither able to do it directly fom nginx (passing caddy) nor coming over by nginx over caddy to opensign completely.
i am able to login, but as soon as i do something with a document (sign or create a template) i get an error message "something went wrong, please try again later".

my domain: is lets say privatedomain.com, and i want to use subdomain sign for this.
my local server-fqdn is test.local

PUBLIC_URL=https://sign.privatedomain.com/
GENERATE_SOURCEMAP=false
REACT_APP_SERVERURL=http://sign.test.local:3000/app
REACT_APP_APPID=opensign
REACT_APP_GTM=GTM-123N7LD

HOST_URL=http://sign.test.local

APP_ID=opensign
appName=open_sign_server
MASTER_KEY=xyz
MONGODB_URI=mongodb://mongo-container:27017/OpenSignDB
PARSE_MOUNT=/app
SERVER_URL=http://sign.test.local:3000/app

that was working with caddy locally, but not with my public fqdn, so i wanted to expose that service for some colleagues temporarely ti the internet. but with this configuration it is not working.

any ideas?

kind regards,
andre

[astrakid]

i guess it is because the documents seem to be referenced by https://sign.test.local/app/files instead of https://sign.privatedomain.com/...
any idea what i have to configure in nginx to get that corrected?

good request in chrome logs (before the error appears):

bad request: