On rare occasions, the PA license file can become corrupt. I think this might have happened to you, so I'd recommend that you delete your license file.  Don't worry, this will not cause you to lose your licenses.  It will just cause the plugin licensor to run the next time you use your plugins, and then it will regenerate your license file.  Just go to the Finder and click on Go > Go to Folder, and the enter this file path exactly as written below:

~/Library/Application Support/

Then look through the resulting directory, find the folder named “Plugin Alliance” and throw it away. Then go ahead and start up ProTools and try to authorize the plug-in again.