A valid license could not be obteined by the network licence manager

ERROR ON AUTOCAD:
A valid license could not be obteined by the network licence manager.
if you are still unable to access a license, contact your system administrator

SOLUTION
 for users with Windows XP: delete the file CascadeInfo.cas on C:\Documents and Settings\All Users\Application Data\Autodesk\AdLM\CascadeInfo.cas

for users with Windows Vista/7/8 delete the file CascadeInfo.cas on C:\ProgramData\Autodesk\Adlm\CascadeInfo.cas