Solution: In the CAD installation directory (C:\Program Files\AutoCAD 2006), find the file AcSign.arx and delete it directly.
AcSign.arx file is a program signature and network service. Deleting it will not affect the application of CAD software.
I have tried several others: reinstalling, modifying path variables, copying DLL files or deleting acad.mnr files ... not necessarily applicable, so don't bother.