This can be related to incorrectly entering the Rhino 5 key. Try this and contact us if you continue to have problems:

1. Make sure the latest Rhino 5 is installed.
2. Close Rhino
3. Delete the LIC files located in C:\ProgramData\McNeel\Rhinoceros\5.0\License Manager\Licenses\ folder. It will look like this 1234567-8f7f-41da-801d-1c87e32f88f5.lic
4. Run Regedit.exe
5. Browse to HKEY_LOCAL_MACHINE\Software\Wow6432Node\McNeel\LicenseCache
6. Delete all the keys in the license cache
7. Start Rhino
8. Enter your license key
9. Try to validate again. (if you are validating offline, you'll need a new code from our web site)