First of all, the reason why you would be receiving this error is most likely due to modification of the license file. Only a few fields can you change within the license file without it needing to be resigned (such as port, vendor nilm, etc). The sign is what is used for authenticity of the license file.
Secondly, VLM 2.0 and 2.1 are supported in Windows 2003 Server. To get a new license file for a different server, email agreements@ni.com with your server name, volume serial number (for Windows drive), and contract number, specifying VLM 2.1. With this information, they should be able to generate a new license file for you.
Additionally, if you have additional technical questions, I would encourage you to go to ni.com/ask (since the forums are typically more for development products moreso than license management software).
Thanks,
Andy McRorie
NI R&D