When trying to activate the license for Autodesk Infrastructure Design Suite Ultimate 2014, I recieve Error 123 ("The serial number you entered is not valid for the product you are trying to activate. Try again"). I've checked multiple times that this is the correct product serial number. In addition, I have tried getting the .lic file from the other registration route and using that to register but I have been unsuccessful so far.
You installed the product to work in a manner different than what your license type is. In other words, your serial number is for a network license, but you installed as a standalone. This leads to being unable to activate, and error 123. Now the fix is to uninstall and reinstall.