Everything was going fine, then one day, one user can not open AutoCAD. She gets the a valid license could not be found error. The weird thing(s) are that there are plenty of licenses and she can open Revit which using the same license. then I find this thread that tracked the problem down to a corrupt CascadeInfo.cas file in C:\ProgramData\Autodesk\AdLM with help of Process Monitor.
After deleteing it and starting AutoCAD the file was recreated and everything works fine now.
Thank you discussion groups!
Update: Here is a link to the Autodesk fix. It is more detailed.