Tenor A800 SIP registration problem

Hello everybody,
my A800 using SIP fails registration – it is also not trying to send any network packages during registration.
The error messages from the eventlog are :

SPROTO : 9311258:[sess]:bbba04 RegisterSession::ProcessTimerExpiration() called

SPROTO : 9311258:[trans]:bbb004 ClientTransaction::ClientTransaction() failure encoding message
SPROTO : 9311258:[trans]:bbb004 Transaction::SendRequest()failure cause code(1)
SPROTO : 9311258:[trans]:bbb004 ClientTransaction::start() failure sending request
SPROTO : 9311258:[ua]:Received Sip Iuca Message: 0x3b

SPROTO : 9311259:[sess]:TransactionAbort – in RegSession

SPROTO : 9311259:[ua]:UA: Registration failed – no action taken now..

Any help would be greatly appreciated.
thank you,
Adi Pop

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More details
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Quintum Tenor A800 Rev. C, loaded with the followind software:

System Software Version: P5-2-1(LEC) (1678285/0xFF74)
Boot Software Version: P4-1-3 (180592/0xE814)
Database Version: 2.08 09-13-2000 (278376)

After loading the firmware, I made a factory reset and configured the following:

1. set the IP address:
IP Address: 192.168.1.3
Subnet Mask: 255.255.255.0
Default Gateway: 192.168.1.1

2. switched to SIP
Unit: 1
IP Address = 192.168.1.3
External IP Address = 0.0.0.0
Name = tenor
Voip = Sip(1)
Online = 1

3. configured basic SIP registration & proxy
Primary Registrar = 192.168.1.1:5060
Primary Registrar User Name= tenor
Primary Registrar Pwd= tenor
Primary Proxy = 192.168.1.1:5060
Primary Proxy User Name= tenor
Primary Proxy Pwd= tenor
…………………………..
Time to Live = 10

I set timetolive to 10 seconds on purpose to force reregistration

The 192.168.1.1 machine is an asterisk. The tenor and the asterisk can ping each other and there are
no firewall rules on the asterisk machine.
Verified with tcpdump and no packets are sent from the tenor during the registration process