For the record: as the PBX was up for ~9 months, I simple restarted it. This cleared the issue.
The log was not really helpful in this case, see:
[8] 20140722141631: HTTP/NTP/SNMP client waited for 60 s [5] 20140722141657: Identify trunk (IP address/port and domain match) 4 [5] 20140722141837: Last message repeated 4 times [3] 20140722141837: DoS protection: Not accepting more calls [5] 20140722141857: Identify trunk (IP address/port and domain match) 4 [8] 20140722141927: Trunk 4: Preparing for re-registration [8] 20140722141927: Trunk xxx: Sending registration to xxx.xx [8] 20140722141927: Answer challenge with username xxx [8] 20140722141927: Trunk 4: setup callback to send re-registration after 240 seconds [5] 20140722141927: Identify trunk (IP address/port and domain match) 4