Apparently the serial code is needed for online play, even if you're using a NoCD.
If you want to do a more thorough test of the NoCD's, you'd have to uninstall and reinstall the game to get rid of the patch. There are different NoCD's for different patch levels. But it sounds like that wouldn't help with the key problem anyway if you're going online.
If you want to enter the key manually, the key is mentioned in this threadhttp://forum.spellforce.com/showthread.php?t=60585
the key registry is stored in HKEY_CURRENT_USER\Software\Phenomic\Spellforce2, click on it and you can edit your key manually, or delete it and reinstall the game.
It's probably best to remove any NoCD's you might have applied before entering the code.
Which version of the game do you have? Is it the Order of the Dawn or the "Diamond" version or a different version?