1º.- As @Quazz said, use the Reset Encryption Data Button for the host/group in UI.
2º.- Delete the token.dat file on the Fog Client folder directory if it exists.
3º.- If you install Fog Client on the image be sure to image the computer with that file deleted, better if you automate the client installation on the first boot.
Make your install script with this recommended commands:
msiexec /i "C:\NewClientInstaller" /quiet USETRAY="0" HTTPS="0" WEBADDRESS="YourFOGAddress" WEBROOT="/fog" ROOTLOG="1" sc.exe config FOGService start=delayed-auto sc.exe failure FOGService actions= restart/60000/restart/60000/restart/ reset= 120 net start FOGService