I installed 10.2.1. agent on a fresh Windows XP SP3 installation but that
device name was already in ZCM database.

I manually deleted the device in ZCM (without doing zac unr) and now, when I
try to register that device, I receive that error:

Server code -34 Invalid device authentication information

How can I resolve this ?

I did a zac cc, zac fsg without any success.