[asterisk-users] ? chan_sip.c:12687 check_auth: Correct auth, but based on stale nonce received

[Brian]

Scenario:
[Asterisk Server] on routed/public IP 
\/ /\  \/  /\  \/  /\  \/  /\  \/
[Draytek Router] --> internal IP's
\/ /\  \/  /\  \/  /\  \/  /\  \/
[internal Network 192.x.x.x]
[IP10s] + [IP10s] + [Softphones]

Everything is good as long as only *1* phone is registered from the
internal network. The minute another phone goes online registration is
dropped and the sip debug complains:

[Feb  9 18:11:00] NOTICE[10381]: chan_sip.c:12687 check_auth: Correct
auth, but based on stale nonce received from '<sip:551 at x.x.x.x:5060>'

This has started since I upgraded from 1.6.1.x to 1.6.2.1. Any ideas how
I can troubleshoot this?