In the following locations:
Delete fencesSig2240.bin (if it's there)
Delete the cache.dat and license.sig
And delete the C:\ProgramData\Stardock\Fences folder
After, try activating again and let us know if you are again prompted.
After successfully activating, please verify that the file C:\ProgramData\Stardock\Fences\license.sig exists. And if you experince the problem check to see if the license.sig file exists again? If it has disappeared there is a possibility that your anti-virus may be quarantining the file.