The second way is to remove completely and use it again. Make sure that the licensing should be happening perfectly. Your problem will definitely resolve.
Please do not say Thank You. Just hit helped me tag.
That's usually a problem with the license demon being too old for the version of Calibre. Do you have multiple Calibre versions installed? You'll want to be sure that your LM_LICENSE_FILE or MGLS_LICENSE_FILE is pointing to the correct license server for Calibre v2012.4.