This article is relevant for all C++ products
Write operations to some third party servers get rejected with the status "EnumStatusCode_BadWriteNotSupported".
Not all OPC UA Servers support writing timestamps. This is not a bug of the Softing OPC C++ UA SDK or the third party Server, it is the official way of a Server to say, that it doesn't support writing timestamps.
Don't call DataValue::setServerTimestamp() and maybe not even DataValue::setSourceTimestamp() on the DataValue that shall be written, respectively call these methods with an empty DateTime variable to clear existing timestamps.