Desktop Manager is a lot less temperamental for activations than OTA is - my guess is that there was a problem getting the ETP.DAT message from the handset into your mailbox (often caused by AV/AS software protecting your mailbox) and as a result it wouldn't have been able to verify the handset was genuine, hence no activation.
DM works out the encryption with the handset automatically so that's probably why it worked
Only a guess but 9 out of 10 failed activations for me are due to that reason