I think this is a permissions issue. When you export the report I have a feeling it writes the file out to the file system and then you can't overwrite it if you run it again. On the library folder, just temporarily try giving Everyone full permissions on the folder and reboot and see if that solves it. If it does, either leave it like that or remove the permission again and then try and sort the permissions out for the IUSR account for that folder.