log4net and Medium Trust
There are many resources on how to deploy your web application to a medium trust environment. MSDN has this article How To: Use Medium Trust in ASP.NET 2.0.
If you use log4net, you already know it's a great logging tool. But log4net is not exempt from its own issues. Phil Haack had issues with it, three years ago. His article(s) is(are) here
But my issue came down to the conversionPattern being applied. So I stripped them out, one by one, until I found the culprit and the formating appeared correctly. Further research found that others have encountered this also. Here's one example: Joe Audette
So, if you're targetting a medium trust environment, don't leave it until the end of the development lifecycle to lock-it-down. Change your development machine's config to match that of your production environment before you start!