@girish I did something slightly different (but maybe not SO uncommon). I created the server and put the clients domain into my own (developer) cloudflare account. After everything worked I created the domain in the client's cloudflare account and changed the NS entries (basically this: https://developers.cloudflare.com/fundamentals/get-started/basic-tasks/manage-domains/move-domain/).
the situation was now, that the API key was still valid but changed the wrong domain entries at cloudflare. so, I imagine cloudron creates the NS entry, checks if this works (which it doesn't) -> brain freeze. Something like this?