ERROR: Couldn't connect to the collab server. Please reload the page and try again.

[iamfoolberg]

Hi there, this is really a nice work.
I build all images and compose the containers, and i can run the excalidraw in my browser.
But when clicking collaborating, the error is:

ERROR: Couldn't connect to the collab server. Please reload the page and try again.

And i found a HTTP GET is issued:

https://www.mydomain.com:8344/undefined/collab-server

Here is the .env file:

# Docker local repo base
BUILDREPO=localbuild

# Timezone
TZ=Asia/Shanghai #Europe/Berlin

# Public server name
PUB_SRV_NAME=www.mydomain.com

# Encryption settings
# Activate acme.sh for certificate creation
# if you diable this, it will generate a self-signed cert
ENABLE_LETSENCRYPT=0
LETSENCRYPT_DOMAIN="${PUB_SRV_NAME}"
LETSENCRYPT_EMAIL="admin@mydomain.com" #"admin@${PUB_SRV_NAME}"

# Use Let's Encrypt staging server for testing
LETSENCRYPT_USE_STAGING=0

# Ports from outside
HTTP_PORT=80
HTTPS_PORT=8344 #443

# Define a proxy server for building Docker images
# BEWARE: you might have to define a systemd service override also!
#PROXY=http://<user>:<password>@proxy.serv.er:<port>

# URLs to access JSON sharing backend
# BEWARE: if you changed the outside ports from standard HTTP/HTTPS, you have to add it here, too
REACT_APP_BACKEND_V1_GET_URL=https://$PUB_SRV_NAME:$HTTPS_PORT/api/v1/
REACT_APP_BACKEND_V2_GET_URL=https://$PUB_SRV_NAME:$HTTPS_PORT/api/v2/
REACT_APP_BACKEND_V2_POST_URL=https://$PUB_SRV_NAME:$HTTPS_PORT/api/v2/post

# URLs to socket server
# BEWARE: if you changed the outside port from standard HTTP/HTTPS, you have to add it here, too
REACT_APP_SOCKET_SERVER_URL=https://$PUB_SRV_NAME:$HTTPS_PORT

# Firebase configuration
REACT_APP_FIREBASE_CONFIG=
#REACT_APP_FIREBASE_CONFIG='{"apiKey":"AIzaSyAd15pYlMci_xIp9ko6wkEsDzAAA0Dn0RU","authDomain":"excalidraw-room-persistence.firebaseapp.com","databaseURL":"https://excalidraw-room-persistence.firebaseio.com","projectId":"excalidraw-room-persistence","storageBucket":"excalidraw-room-persistence.appspot.com","messagingSenderId":"654800341332","appId":"1:654800341332:web:4a692de832b55bd57ce0c1"}'

# excalidraw-json S3 backend configuration
EXCALIDRAW_S3_ENDPOINT=http://minio:9000
EXCALIDRAW_S3_BUCKET_NAME=excalidraw
EXCALIDRAW_S3_ACCESS_KEY_ID=minios
EXCALIDRAW_S3_SECRET_ACCESS_KEY=minioKey1234
EXCALIDRAW_S3_FORCE_PATH_STYLE=true
EXCALIDRAW_ALLOWED_ORIGIN=''