Yikes , this is not good and looks like your nginx configuration is broken. Did you set up everything as shown in the tutorial, including the proxy_set_header directives?
This is fine if and only if you follow this howto:
Discourse needs to know it is running in a subfolder or all links will be broken; the tutorial above configures this.
If you follow the combination of this and the original howto, you should have a working Discourse as long as you stop Apache and start nginx for a moment. If this is not the case, can you post your nginx configuration files?
Iām not sure all of this works under nginx (as ownCloud needs PHP, which is not my forte). Setting up nginx to route requests to Apache should be relatively easy, I have a working setup using this apprach.