Hi Seb,
thanks for the quick and thorough reply and thanks for confirming the fishiness of the guide. Thanks also for explaining the way it is supposed to work.
After some more troubleshooting, I am sure that there is something wrong with my Apache config but I have no clue how to fix this, Apache seems to ignore my vhost-setup and I have no idea why:
When I connect on 127.0.0.1:80, I now no longer see the default page because I copied all the document root and directory permission stuff from the httpd-vhosts.conf to the httpd.conf.
There are no errors in the error.log. In the access.log, however, I see the request “Get /visualizer.js HTTP/1.1" 200 saying everything works as expected, but afterwards there is a “Post /paraview/ HTTPS/1.1" 404 207 telling me, Apache does not find the /paraview subpage. According to the vhost config, Apache should forward this to http://localhost:9000/paraview which apparently does not work. Also, no session is started when I access on port 80 (because obviously the forwarding to :9000 does not work). When I access on port 9000, I see the session log (the weird name file you mentioned). So both parts work individually, but the connection between the two is broken despite Apache being configured accordingly. Any idea how to get Apache to do what it is configured to do on Windows? On Linux, I know there is a2ensites and restarting Apache service is always a good idea, but on Windows I am sort of lost.
Slightly off-topic: I am not bound to Windows. I started setting up ParaViewWeb on Ubuntu 18.04 but could not get that to work either, because I failed to install the opengl drivers which probably gave me error messages when running the launcher script. Since the deployment guide on the website is for Ubuntu 14, I figured there was some fundamental issue I wasn’t going to solve anyway, so I switched to Windows. If you are more comfortable debugging issues for Linux, I will try that again and report back…? Can’t imagine I am the only one having issues setting things up (but maybe I am 
)
Dan