The server block per default requests (with 403) to the same machine / local network.
Then "Blocked access to local network: ..." should be in the server log.
Modify the file C:\ProgramData\Qlik GeoAnalytics Server\server3\modules\webmap5\config\WebMap5Module.xml
and restart server.
<?xml version="1.0" encoding="UTF-8"?>
<WebMap5Module allowGetResourceLocal="true" />
Force QGA Server to set https as referal protocol
When doing port forwarding or using load balancers in front of QGA server(s) then it's necessary to tell the QGA server to use https as the protocol in subsequent calls. The protocol behind the LB is normally http.