The key to understanding this error are the words, "system clock has been set back". The Ansys licensing system goes out and looks at the timestamps of a bunch of files on the computer. If it finds a file with a timestamp that is IN THE FUTURE, then it assumes that the system clock has been set back to a time IN THE PAST in order to get around the expiration date on the Ansys license.
The corrective action is the remove any file with a timestamp in the future. One sure way to clean up the computer is to format the C: drive, reinstall Windows, and reinstall all your programs. That is a huge job.
A Google Search has some links to users who have experienced this error before.