The other thread on this appears to be completely inaccessible for some reason, so I will post this here for the benefit of anyone else who encounters this issue.

After upgrading to v5.2.1 three of my production websites started getting flooded with errors in the Event Log all related to the PurgeModuleCache scheduled task.  By the time I noticed it three days after the upgrade, I had in excess of 17,000 exceptions in my site log as this task runs once every minute and would generate 4 log items every time it failed.  The crux of the error was that the process did not have permission to access "/Cache/Modules/".

The resolution for me was to ftp in to the website, and then delete and recreate the /Portals/0/Cache/Modules folder.  Obviously if you are running multiple portals on your DNN instance you would need to do the same thing for each of the portals.  I did this on all three production websites that were having this issue, and the errors stopped immediately.