This warning is typically caused by a on object or type reference which points to a missing or no longer existing class. To find the object, use the desktop's advanced find window and type the following query:
node[id = <id>]
where <id> is the ID reported by the warning message.
Then navigate to the object via the context menu in the result table.
You can also supress this warning by setting a ToolInfo variable:
Starting with OpenAmeos 9.2.3, this message has been changed to
Error in model: object instance scope '...' has no corresponding class definition