Hi Björn,
Sorry for the issues, we are working on improving our solution for better user experience. In the meantime, here are few things that we can try. You can try the SecondScreen application from your PC to make sure this is not a software issue.
To open the SecondScreen application from the PC, first click on the QR code in the pairing dialogue. This will open IE11 (this is fixed in future versions, and will open the default browser), from there you can copy paste the URL to your browser (chrome, firefox or edge). You should then be able to see the empty project ( or the project you have opened ).
If this works, then the issue lies in the network configuration. Check if you don’t have AntiVirus with firewall capabilities that could block the incoming connections. It could also be the case that your router for some reason blocks or reroutes certain ports on the local network. This can be checked in your router administrating page, usually under port forwarding settings. It could also help to set forwarding from the iPhone to the PC. Does the browser provide any error information, why it is not opening http connection?
Thank you, best regards
Jan