Sofia-SIP throws "Internal Server Error"

Phone -> Accounts -> SIP
Username: 3450@192.168.2.92
Password: xxxxxxxxx
Advanced Settings:
User for telephone numbers: Checked
User name: 3450
Transport: Auto
Outbound proxy: 192.168.2.92
Port: 5060
The rest: Default

The phone registers fine with asterisk. However, when trying to make a call from another SIP phone to the N900 I get:

SIP Phone N900
------------------------
Invite with media description =>
100 Trying <=
180 Ringing <=
500 Internal Server Error <=

I have played with the settings like crazy, but nothing seems to help. Any other SIP phone works fine with my Asterisk (Cisco 9640, X-lite ....)

Did anybody else succeed in using the N900 with Asterisk?

Cheers,

// Jonas