Debugging a Data Execution Prevention (DEP) failure in an Excel addin

I am developing an Excel addin that provides specialised calculations. VBA code in the addin talks to a COM server written in Python. The communication uses strings or Variants.

Someone using the addin has reported a failure three times now (Office 2010-32 bit, Windows 7 Professional) on two different machines. On each occasion, they turned off the computer and when they turned in on again, the addin was disabled by Windows Data Execution Prevention.

I would like to try to track down the reason for this, but so far have not been able to reproduce the problem. I have even tried one of the machines with the same spreadsheet, but did not manage get the addin disabled.

Can anyone tell me whether this action of disabling the addin by Windows only occurs at shutdown/restart, or can it occur at any time? Not only was the addin disabled, Windows also deleted a number of the DLLs needed by the COM server!

I know that the spreadsheet had been worked on for several days without apparent problems before the machine was turned off. So should I consider that the problem might have occurred at any time while the addin was being used, or does the failure at shutdown/restart point to a problem setting up, or tearing down, the components involved?

Any suggestions on how to approach this problem?