Grandstream GXP 2000 won't register on FreePBX

I’ve a strange problem.
I’ve just configured a new FreePBX server and installed the free OSS PBX End Point Manager - last version.
I’ve two Grandstream phones:
N. 1 BT-200
N. 1 GXP 2000
I’ve installed the last firmware package available via End Point Configuration Manager and added - via MAC addres - all two phones (each with its extension number).
Then on each phone I’ve configured “Firmware Server Path” and “Config Server Path” so that each point to FreePBX server IP address.
All two phones - when rebooted - download the last firmware and the right account setting (extension number, server IP address, NTP server and so on…).
But one - the BT-200 - is correctly registered and the other - the GXP 2000 - won’t register in any way.
The problem is that the second phone (GXP 2000) don’t provide the right password to the FreePBX server (if I watch log files relative to its extension I can see “Wrong password”) and, because I’ve installed and configured Fail2ban, the IP adrress of the GXP 2000 phone is constantly banned.
It seems to me that I’ve followed the same steps for all two phones but one is registered and the other no.
Any idea?
Thanks in advance