this is hard to say with such little information. The OpenSecureChannel is not working.
The third pary client is from which company?
Is that available for testing?
Have you been able to connect to any other server with this third party client?
Have you tested with UaExpert? Does it work when using UaExpert and doing similar scenario?
Our educated guess is, that the client is using the wrong (or no) security algorithm (or padding) for encryption, hence the cypher is of incorrect length, and rejected by the server.