Then they read the STRING in a wrong way. The two initial bytes is part of the format for a STRING and should be interpreted properly by the OPC server. The two bytes cannot be ignored.
Do they read as a STRING or as an ARRAY of CHARs ? If the latter then it explains it.
You can send and they can read as an ARRAY of CHARs instead of a STRING, but that only works if the text to transfer always has a fixed length.