ntpd: problem syncing with ntpdate

[Michael Adam <openbsd_(_at_)_obnox_(_dot_)_de>]

Hi,

I have a box with OpenBSD v3.6 (stable) and with ntpd running. 
Now, I cannot get the time from this server with ntpdate: not
from a linux box, and not from the same box with the ntpdate
from the ntp package. rdate from OpenBSD works, though.
The firewall is not a problem, all packages get through nicely:

openbsd/192.168.6.33:~# tcpdump -npi rl0 port ntp
13:25:11.304519 192.168.6.50.123 > 192.168.6.33.123:  \
    v4 client strat 0 poll 4 prec -6 (DF)
13:25:11.306097 192.168.6.33.123 > 192.168.6.50.123:  \ 
    v4 server strat 2 poll 4 prec 0
13:25:11.306355 192.168.6.50.123 > 192.168.6.33.123:  \
    v4 client strat 0 poll 4 prec -6 (DF)
13:25:11.307897 192.168.6.33.123 > 192.168.6.50.123:  \
    v4 server strat 2 poll 4 prec 0
13:25:11.308053 192.168.6.50.123 > 192.168.6.33.123:  \
    v4 client strat 0 poll 4 prec -6 (DF)
13:25:11.309583 192.168.6.33.123 > 192.168.6.50.123:  \
    v4 server strat 2 poll 4 prec 0
13:25:11.309724 192.168.6.50.123 > 192.168.6.33.123:  \
    v4 client strat 0 poll 4 prec -6 (DF)
13:25:11.311240 192.168.6.33.123 > 192.168.6.50.123:  \
    v4 server strat 2 poll 4 prec 0

but ntpdate just says:

linux/192.168.6.50:~# ntpdate -v 192.168.6.33
 9 Mar 13:23:30 ntpdate[27382]: ntpdate 4_(_dot_)_2_(_dot_)_0a_(_at_)_1:4.2.0a+stable-2-r \
    Sun Jan  9 16:13:28 CET 2005 (1)
 9 Mar 13:23:30 ntpdate[27382]: \
    no server suitable for synchronization found

Has anyone experienced similar problems?
Any hints are appreciated!
Thanks in advance,

Michael