Thanks for posting on PI Square.
Looking at the issue, my initial inclination is that the version of ActiveView (which contains the processbook plugin) is not compatible with the version of Processbook.
Would it be possible for you to downgrade the version of Processbook to the version of ActiveView? The latest version of ActiveView is 2014 or 3.4. Please try downgrading Processbook to match (2014 or 3.4).
Please let us know if this clears the error.
Thanks for the reply - I am confused about your suggestion, though. ActiveView was not installed. I downloaded version 188.8.131.52 from here and installed it but that did not help. https://techsupport.osisoft.com/Products/PI-Visualization/PI-ActiveView/
Unfortunately the customer is not interested in downgrading (even though our add-in works fine in ProcessBook 2015 - 184.108.40.2064).
Update: it turns out that even though the following line itself did not error, removing it and instead using local PISDK.PISDK objects in the various functions is what got rid of the popup message.
Private pPISDK As New PISDK.PISDK
Edit: reverting the references back to the original DLLs was also possible.