ok, I could solve it (the error is still very unhelpful), the problem was capitalisation on the dictionary keys, here's the correct one:

Config = new Dictionary<string, Dictionary<string, string>[]>
                {
                    {
                        "bindings", new[]
                        {
                            new Dictionary<string, string>
                            {
                                {"authLevel", "function"},
                                {"direction", "in"},
                                {"name", "req"},
                                {"type", "httpTrigger"},
                            },
                            new Dictionary<string, string>
                            {
                                {"direction", "out"},
                                {"name", "res"},
                                {"type", "http"}
                            }
                        }
                    },
                },