I would try re-upload the one that shipped with mojoPortal. Possibly the file was corrupted when uploaded previously and re-uploading it might fix it. Or file permissions may not be correct. Whatever user is the identity on the application pool must have read access to the entire web including all files in the /bin and write access to /Data and /App_Data. If permissions are not correct on the file it can fail to load it.
I have no other ideas, no-one else has reported any problem such as this and I don't believe it is a problem with mojoPortal, it something strange about your environment or configuration.