I have just installed the latest trunk to check out ipv6 functionality.
I am falling at the first attempt already: Adding a new subnet. I have tried several notations but they all seem to fail. Am I doing something wrong or is that specific feature not ready yet?

You would need to install the code in the ona-ipv6 branch. You can reach that here: https://github.com/opennetadmin/ona/tree/ona-ipv6. I am still working on some install details but I believe it to be functional. Do give it a try and see how it goes.