Okay, so now am free to investigate this problem more.
The external address is one of our monitoring services. It just connected without asking anything and disconnected directly. This has now been fixed.
Still, am having this error left in the logs:
Jun 24 13:30:01 websrv1 mydns: read_tcp_length: no data for fd 10: 127.0.0.1: Client closed TCP connection
The above error message repeats itself every 10 minute, exactly. Like timestamps 12:10:01, 12:20:01, 12:30:01, 12:40:02 and so on.
So how can I solve this? Is this a bug in ISPConfig 3's monitoring service?