Terminal CALS License upgarde issue !!!!!

Hi Tech team,

  I have used a Terminal CALS License more than 50 users in windows 2003 std Edition. I upgrded the w2k3 to win 2012  terminal CALs license, it's worked normaly more than 120 days . But i try to connect the RDP logins ,Its shows error like "  The remote session was disconnected because there are no Remote Desktop Licence Servers available to provide a license. " Please advice on this issue

Thanks ,

VARUN