What I am wondering is, that I see the internal address behind the reverse proxy in my Browsers address bar. In theory my browser should not know this, as far as I understood the functionality of the reverse proxy.
I figured out, that I also had no trailing / at the end of the Reverse Proxy configuration.
After I added this, something is happening, but ends up in a blank page. The access.log shows, that there is some traffic and some files are loaded.
If I have a PC in the same LAN as the Reverse Proxy and try to open the Portal via Reverse Proxy, I have the issue. If I use the server instead, it works. That eleminates everything else in the network (i.e. the Firewall between DMZ and internal network).
In the next step I investigated the website with the debugger of the browser. I figured out if I call http://nethserver/portal, there are some 404 errors:
Maybe again a tailing slash missing...? Yepp...
Never the less: The application itself cannot be accessed. Do you know how the Reverse Proxy is reacting to HTML5?
In the next step I tried the same from external with Hostnames instead of internal with IP.
If I use http://mycompany.com/portal, like internally, I get an empty page with 404 errors. If I use http://mycompany.com/portal/ it results in the same end like internally.
I will ask the provider of the appliance what exactly they are using. Except of "HTML5" the engineer I talked to told me...