Hi.  This is a long shot, but this error looks like it might be permissions related.  I've had various strange errors with this and other modules, and I've been able to solve some of them using the FileMon utility from www.sysinternals.com to track down the permissions issue.  Filemon logs all file activity occurring on the server.  It shows which process is accessing which file using what type of operation (read, write, etc.).  I run it from the server console as "administrator".

 This procedure will work if you own your own DNN Server.

1. Download Filemon to your DNN Server.
2. Run Filemon in capture mode on your DNN Server.
3. Perform whatever action generates the error message or condition.
4. Stop the capture in the FileMon Utility.
5. Look for any entries in the FileMon log that report "access denied".
6. Assign appropriate file permissions to get rid of the errors.
   

I've run into several issues where the ASPNET account didn't have appropriate permmissions to some system file.  The kicker is that these errors showed up after I applied all of the patches available from Microsoft on Patch Tuesday.  Apparently, some of the updates from Microsoft monkey around with existing permissions in the name of security, and occassionally break existing functionality.  I've had this happen twice due to Patch Tuesday updates.

I realize that you simply upgraded to DNN 4.3 instead of applying Microsoft updates, but it still might be worth checking out.

Good Luck!
Van