Make sure that the domain you're accessing the Wiki through is setup in /etc/hosts (for Linux users).
Example:
If your wiki is located at http://deki.brandnewwiki.com, then add the domain to /etc/hosts as follows
127.0.0.1 deki.brandnewwiki.com deki
One thing to confirm is that you can get to the actual API e.g.
curl http://localhost:8081/host/test
Get that working first ... if it's not working then the dekiapi isn't running for some reason.
Once the localhost:8081 call works, then try
curl http://yourwiki.yourdomain.com/@api/host/test
if that doesn't work, then you need to get your mod_proxy working.