One hunch I have would be that you do not run the API in threadsafe mode (the queue mode, where you poll the API for responses in e.g. FixedUpdate)
Unity is not thread safe, and the API uses a separate thread to handle the network communication.
Basically all Unity usage of the SFS1 API should run in the queue mode. SFS2X defaults to this - but SFS1 doesnt due to historical reasons.