Just a small follow-up, I managed to get this link working by debugging the connection on OpenOpcUA side and comparing with a Discovery example from the unified automation toolkit.
The only difference I could find in the GetEndpoints request was a nNoOfProfileUris parameter that was -1 in the ignition request and 0 in the unified automation request.
This difference causes OpenOpcUA to return an empty list of endpoints.
After slightly modifying the code to do the same for -1 as for 0, the OpenOpcUA server correctly returns 3 endpoints an no longer prevents Ignition to connect.
I still need to investigate if there is another underlying problem that I am just hiding with this workaround but I hope Michel can shed some light on this as well.