This Autodesk Knowledge Network [AKN] article describes the issue and provides a procedure to fix the problem. In brief, with the program closed, you delete the damaged WHITELIST.XML file, reboot your computer, and then restart AutoCAD Architecture or AutoCAD MEP. Turn Desktop Analytics and the Customer Involvement Program features back on, if you turned them off, by selecting the right side (down arrow icon) of the Help split button, and choosing the Desktop Analytics and Customer Involvement Program items (one at a time). After several minutes, the WHITELIST.XML file should be downloaded from the Autodesk website (if you have an active internet connection). This file should not be damaged, and you should not experience the Exception in mc3.dll / mc3d.dll ARX Command error message.
An alternative solution to stop the crashing is to use those menu items to turn off both Desktop Analytics and the Customer Involvement Program. The AKN article describes how to do so via editing the registry if you are unable to open the program and get to the Help Menu.
- In order to get the full Help drop-down menu, you need to have at least one drawing file open. Just having the Start tab open will result in a truncated menu that does not include the Desktop Analytics and Customer Involvement Program items.
- As of November 7, 2015, the AKN article lists this folder as the location of the WHITEPAPER.XML file for AutoCAD Architecture 2016:
I had that folder, but there was not a WHITEPAPER.XML file in it. I found it in this folder, instead:
The difference between the two is the product version number folder. I had SP1 for AutoCAD Architecture 2016 installed, and 126.96.36.199 is the version number after SP1 is installed. You may want to check the About AutoCAD Architecture 2016 dialog (last item on the Help drop-down menu) to see what your version number is, and look for the WHITEPAPER.XML file in the folder with that number.
- After not finding the WHITEPAPER.XML file in the ...\188.8.131.52\MC3 folder, I tried searching my entire C: drive for WHITEPAPER.XML. Windows will apparently not search inside all (any?) of the User folders when the search is started at the root of C: drive, as I got no hits when doing so. After I saw the file in the ...\184.108.40.206\MC3 folder, I tried searching from the C:\Users\[user]\AppData\Roaming\Autodesk folder and then it found the file.