The "System clock has been set back" variation of the "Invalid or missing license file" errors usually occurs when the computer’s system clock has been set forward and later corrected, but it could also be caused by a file with an incorrect time stamp on the computer. FLEXlm (the licensing software used by Maple) is throwing an error because it interprets the presence of future-dated files as an attempt to circumvent the license expiration date.
There are likely files present on your machine that have time-stamps that are in the future compared to the system clock.
After correcting the system clock (if necessary,) these files can be searched for, then re-saved so that their time-stamps match the system clock time. Please ensure that the files in question are not system-critical files before modifying them.
The files that must have correct dates for the license to be verified can not be predicted. Correcting the dates of all future-time-stamped files should resolve the issue.