Hello,
Is that ReadItem/ReadMultipleItems or ReadItemValue/ReadMultipleItemValues?

Assuming that you are calling ReadItemValue/ReadMultipleItemValues:
These methods return just the actual Value, while OPC server always supplies a triple of Value/Timestamp/Quality. Since the Value is generally only available when the Quality is Good/Uncertain, these methods set up an additional subscription to observe the values wait until the quality becomes Good or at least Uncertain. If the server does not supply a VTQ with a Good/Uncertain quality within a timeout period, there is no Value we can give you, and you receive this error.

You can use ReadItem/ReadMultipleItems instead. In that case, you receive the whole VTQ - and it is then up to you (your code) to decide what to do if the Quality is not Good/Uncertain and therefore there is no Value to talk about.

Best regards