If the log is empty it means the file system is not writable, it cannot write to the log, it also cannot delete the file, probably both are caused by bad folder permissions.

All files and folders beneath /Data and /App_Data need to be writable by the user that is the identity on the application pool.

Possibly if the site was moved from one server to another it has some funky file permissions that need to be reset on existing files.

Hope that helps,

Joe