The error indicates the url is /Admin/ServerLog.aspx and it looks like that is the first page being accessed right when the application is being restarted.
Does it happen on other pages?
I would keep an eye on the application_start and application_end events being logged, maybe the application is being recycled too frequently.
There is some code in application_start event that is expected to fail under medium trust but it is in an error handler whereas the posted error does not appear to be a handled error. Possibly if the app is recycling repeatedly the error handler fails to catch it.
Hope it helps,