Good day Fred,
Thanks for the inquiry. Can you please provide us confirmation from either the SmarterMail 12x or 13x installation the error logs from the following folder:
[C:\Program Files (x86)\SmarterTools\SmarterMail\MRS\App_Data\Logs]
We would presume that there may have been a specific issue with the upgrade that the error log may point to. If you are in a position to upgrade to a test server please let us know the results. One thing to remember is to check the application pool identity to see if the account, possibly the Network Service has full control over the SmarterTools folders. You may also want to confirm that the Managed Pipeline mode is set to 'Integrated' apposed to 'Classic'
Thank You.