Update: simple solution found
After trying out so much information from the internet, I became more and more confused. Nothing helped or worked. So i started with a fresh installation of OpenProject (don't forget to delete the ~/.bitnami). One last hint i had found.
Mac, where OpenProject ist installed, has IP 192.168.2.40
OpenProject has IP 192.168.64.5
So got to OpenProject Panel Network and "add" a new port forwarding route 8080 to 80 - Thats it.
All other computers in network can reach OpenProject with 192.168.2.40:8080 now.