At first glance, those two messages do not seem to originate from the same source code, as there are only 2 lines between Activate and AddOLEObject.

I'm not familiar with the modules you're using, but still, a possible cause for the error may be related to the files not having 15 charts, and you are looping all the time for that amount of charts. Look for some method that gives you the amount of such elements in a sheet.