DomainKeys issue: no signature in outgoing email headers (Plesk 11/Ubuntu 12.04.2)

[AlfS_ter]

Hello,

I have enabled domainkeys support both server side and in the email settings of my domain, but all outgoing emails are missing a domainkeys signature in their headers like the following:

Domainkey-Signature: a=rsa-sha1; q=dns; c=nofws; s=default; d=mydomain.de; b=E8iksNsSgSK92A0MbIz/PWE5YQ1Bzrv2dNrkPX8cJxl3fYniO00vpsHuuwNTcl7h/WX8A6nmUnU8qe4U8YcFw6sWE8cmeGrkMJwCmEXz2rpPuEYo1Nx/1uof/ILj5Juo;
h=Received:Received:From:Content-Type:Subjectate:Message-Id:To:Mime-Version:X-Mailer;

The headers of my emails show (sent to a yahoo account):

Received-SPF: none (domain of myprivatedomain.de does not designate permitted sender hosts)
domainkeys=neutral (no sig); from=myprivatedomain.de; dkim=neutral (no sig)

I also checked the DNS settings of myprivatedomain.de and, yes, there are TXT records like:

default._domainkey.myprivatedomain.de. TXT p=MHwwDQYJKoZIhvcNAGGFiQADawAwaAJhHFUWHWFWIDJIWDJW ysdv7tBPSnOz3c6lFtnlAWIhE/TvmajIiGICmeVEmqrddN54sn70tsjswDCfZNIdYdQSftXOTvWy GEM7UM7e+rJjVsr/TT4ruoqy9oIxfwIDAQAB;

_domainkey.myprivatedomain.de. TXT o=-

My issue seems to be quite the same as outlined in this thread which has never been solved but by resetting the server:
http://forum.parallels.com/showthread.php?105391-Email-No-SPF-Domain-Keys-Headers-in-emails

Are there any ideas meanwhile to solve this "no signature problem" without reformating the server?

N.B. The domainkeys check of incoming emails works well refering to mail.log.

I am running PLESK Panel 11.0.9 Update #48 on an Ubuntu 12.04.2 LTS system of a virtual server at Strato, Germany

Thanks for any help.

Alf