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

[Jakob Curdes]

Some weeks ago I asked a question on the antispam plugin; obviously 
nobody could help me. I just worked on it again but made no progress.
Is actually anybody out there running the antispam plugin with dspam?? I 
am willing to write a Wiki page for configuring this as soon as I get it 
to work.

The problem still remains and the outlines are:

- antispam plugin is compiled, configured for dspam and debug is 
switched on.
- plugin starts with dovecot and throws debug info to /var/log/messages 
(debug logging activated via syslog)
- as soon as I move a message into the spam folder it gives an error 
message saying "antispam signature not found"

I checked that the given location for the dspam executable is correct, 
that the user executing it (dovecot?) is actually able to execute dspam 
and that dspam trusts this user. I triple-checked that the signature is 
configured correctly. I have lines like

X-DSPAM-Signature: 1,49084a24139132188715614 

in all mails  and my plugin conf says

    # mail signature (used with any backend requiring a signature)

    antispam_signature = X-DSPAM-Signature

I do not get any error messages in the debug output, only dovecot says:
dovecot: Nov 09 13:19:40 Error: IMAP(****): Next message unexpectedly 
lost from 49

Any ideas? How can I get a more specific debugging for the signature? Or 
might this still be a permission problem?

Here is my plugin config:

# antispam plugin - learn spam from folder
    antispam_signature = X-DSPAM-Signature
    antispam_spam = spam
    antispam_dspam_binary = /usr/local/bin/dspam


Regards, Jakob