also-works.cs

// /api/wut
public GetGuidesModule()
            : base("/api")
        {
            Get["/wut"] = p =>
                                {
                                    return "Hello";
                                };
        }

no-work.cs

// /api/export/something
public class GetGuidesModule : NancyModule
    {
        public GetGuidesModule()
            : base("/api")
        {
            Get["/export/{type}"] = p =>
                                {
                                    return "Hello " + p.type;
                                };
        }
    }

web.config.xml

<add key="umbracoReservedPaths" value="~/umbraco,~/install/,~/events-api/,~/api/" />
....
<add name="Nancy" verb="*" type="Nancy.Hosting.Aspnet.NancyHttpRequestHandler" path="api/*" />

works.cs

// /api/anything-here
public class GetGuidesModule : NancyModule
    {
        public GetGuidesModule()
            : base("/api")
        {
            Get["/{type}"] = p =>
                                {
                                    return "Hello " + p.type;
                                };
        }
    }