We need to see if and how the request reaches LS Server.
If the Server is under your control, please check the log. The following log configuration is needed:
LightstreamerLogger.connections at INFO level
LightstreamerLogger.connections.http at INFO level
LightstreamerLogger.webServerat DEBUG level

Moreover, even though, as far as we know, there isn't such a bug in .net SDK 5.1.0, please upgrade to 5.1.8 to simplify the scenario.