ntpd sendto: Can't assign requested address

[feily]

Hello,

I run OpenBSD 5.0 (amd64) with ntpd. About 5 to 10 times a day, it logs 
errors like the following to /var/log/messages and /var/log/daemon:

Jan 11 02:04:53 abc ntpd[26588]: sendto: Can't assign requested address

/etc/ntpd.conf:
listen on *
server ptbtime2.ptb.de
server ptbtime3.ptb.de

This box is part of the ntp pool (www.pool.ntp.org) so it gets about 10 
ntp querys per second on average, occasionally many more.

What to do?
Thanks
feily