Hi Lars,
By any chance do you have a call to UseTransport(...) anywhere? We have seen a similar exception before and it was related to UseTransport(...). Removing that call fixed the issue.
You shouldn’t have to explicitly set a transport, the infrastructure will do that for you. When it does, it intercepts the transport created and adds some additional Azure Functions pieces. When you call UseTransport(...) directly that work is undone and critical pieces of infrastructure are missing at runtime.
Regards,
Mike