AutoCAD LT 2011 activation: serial # not valid

Hi,

After installing AutoCAD LT 2011, I'm unable to activate it using our product's serial number ("The serial number you entered is not valid. Try again. (1)").

We have enough seats (10) purchased so there shouldn't be an issue with running out of licenses. To be safe, I tried using the License Transfer Utility to remove a license off another machine in our office - but I got the same error message about the serial number not being valid. Can someone please contact me directly to see what the issue may be?

Thanks,

Alex