Hello, I believe this happens because dataS should be set to "{}" since the service is trying to parse it as a JSON object. I fixed it in the new version of CaptchaSharp, but it will still take time to release, so this is a possible solution you can adopt in the meantime.
Please let me know if it works, I’m not 100% sure that a JSON string will do, maybe it needs to be an actual object. In that case, I’m afraid you simply have to wait for the new version, which will be included in OpenBullet 2 v0.3.2.