When you started the network version of AutoCAD 2009 or AutoCAD 2010 on a Microsoft Windows Vista or Windows 7 workstation, you received licensing error 1.5.-15.
Basically the solution is to open Local Area Connection Properties dialog box and clear the "Internet Protocol Version 6 (TCP/IPv6)" check box. Hopefully a better solution will come as IPv6 might be needed eventually.
Knowledge Base document has all details: Licensing error 1.5.-15 when running the Network License Manager on Microsoft Windows Vista or Windows 7
Here is another identical error but another solution: AutoCAD 2007 network license manager Error 1.5.-15