Registration error: 408 - Request Timeout

[mbais]

I can't register an x-lite phone from outside.
From inside the firewall check fails with the following errors:

1 9000 Error (4) Global address (returned by STUN) is not accessible from outside. This issue can be generated only if a STUN server is defined for checking. addrFromSTUN = 82.93.164.152:57376

2 9000 Error (6) Different NAT translation for different requests (to STUN and to Firewall Checker Agent). This situation can occur if there is something wrong with NAT translation or a symmetric NAT is used. addrFromAgent = 82.93.164.152:57381
addrFromSTUN = 82.93.164.152:57376

3 9000 Warning (8) Local port is not blocked from outside. STUN server has returned global port different from the local one, but the local port is also accessible from outside.
4 9000 Warning (10) Port number has been changed during NAT translation. externalAddress = 82.93.164.152:57376

My firewall reports that all is well:
{Administrator}[systemlog]=>show
<84> Dec 9 15:36:32 FIREWALL rule (1 of 3) : Protocol: UDP Src ip:
87.230.29.162 Src port: 4200 Dst ip: 192.168.1.78 Dst port: 9000 Chain: forward_host_service Rule Id: 46 Action: accept

<84> Dec 9 15:37:15 FIREWALL rule (1 of 1) : Protocol: UDP Src ip: 213.201.133.242 Src port: 2281 Dst ip: 192.168.1.78 Dst port: 5060 Chain: forward_host_service Rule Id: 45 Action: accept
{Administrator}[systemlog]=>

-> the first entry is the firewall check;
-> the second entry is my x-lite phone connecting to the 3cx server

The x-lite phone seems to be registered on the server, but the extension is unaware (408 error).

What is going wrong with the STUN-test on the server?

Thanks.