If the caller (XBase++) is effectively invalidating the HRESULT-s we are returning, you can use ReadMultipleItems or ReadMultipleItemValues instead of ReadItem/ReadItemValue. The difference is that ReadMultipleItems/ReadMultipleItemValues returns result objects that contain the Exception property, and it in you find the IException object you are mentioning.
ReadMultipleItems/ReadMultipleItemValues does never directly "fail" (return an HRESULT other than S_OK) except for invalid arguments (a programming error) or very rare "catastrophic" conditions.
So, if XBase++ is replacing our HRESULT from ReadItem/ReadItemValue with something of its own, this workaround should prevent it from doing so.
If. on the other hand, XBase++ generates that error on its own, not just when WE return some kind of failure, then you may still be getting the same error with this workaround. Whether in such case the output results objects will contain valid information depends on X++.

I should also add that there is a possibility that XBase++ transforms the error code we generate to this error code; that would mean that the proper error code is "hidden". Do you have a knowledge of how XBase++ handles OLE Automation errors? We can find that out by experiment, but if you knew already it would help.

Hello,
I assume that "-1073446906" mentioned below is the error code you are getting. This code (corresponds to 0xC0048006 hexa) is not directly generated inside our OPC client components. (I have checked the list of codes we use). It can therefore come either from the target OPC server itself, or (less likely) from supporting system calls that we need to make (such as for manipulating VARIANTs etc.).
First thing I would do - ask the server vendor (Siemens) if their OPC server ever generates this error code, and under which conditions.
It the above isn't possible or does not lead to conclusion, we would have to put some more advanced diagnostics in place. This can be either in form of Debug build of our components, or by an OPC Analyzer (a tool from OPC Foundation) that is OPC Client/Server that you can put "in between" your app and the target server, and have it log the OPC calls being made. Both these options mean extra work, so please first let me know whether you can get some info from Siemens.
Best regards
Zbynek Zahradnik