@pitaj damn.. that fixed my problem. I didn't know that one has to write the 1:1 url you call in your browser as the config.json url setting..

So instead of:

{
"url": "http://localhost:4800",
"secret": "XXXXXXXXXXXXXXXXX",
"database": "mongo",
"port": 4800,
"mongo": {
"host": "127.0.0.1",
"port": "27017",
"username": "nodebb",
"password": "XXXXXXXXXXXXXXXXX",
"database": "nodebb"
}
}

I wrote:

{
"url": "http://MYWEBSITE.COM",
"secret": "XXXXXXXXXXXXXXXXX",
"database": "mongo",
"port": 4800,
"mongo": {
"host": "127.0.0.1",
"port": "27017",
"username": "nodebb",
"password": "XXXXXXXXXXXXXXXXX",
"database": "nodebb"
}
}

if you use https, of course the url has to be https://...
1:1 as if you would call it in your browser. thank you very much!