[Dovecot] antispam plugin claims "antispam signature not found"

[Jakob Curdes]

Hi,

I am trying to setup the antispam plugin using the direct dspam backend.
I have set

antispam_signature = X-DSPAM-Signature

in dovecot.conf
and in all mails I have signature lines like

X-DSPAM-Signature: 1,49084a24139132188715614

but still, if I try to move a mail into the spam folder, i get the IMAP error message

"antispam signature not found".

I thought it might have to do with permissions and made sure that the user running dovecot is able 
to execute the dspam binary.
I also tried to enable debug logging to syslog (via the .config file in the make process) but I do 
not see any debug messages showing up; where should these go? to the mail facility?

Perhaps somebody who has a working setup can shed some light on this?

Yours,
JC