DKIMPROXY signing issues


we are using DKIMPROXY 1.2-3 for signing our outbound mails on Ubuntu 10.04. I setup this some month ago and at this time I was able to create my own test mails and send it to DKIM testers with correct results.
Setup is like:
Mail Client -> DKIMPROXY.out -> postfix -> INTERNET

I wrote a test script which creates an email and sends it to dkimproxy. (see attachment)
If I send an email to a tester I got a failed response

dkim=fail (body has been altered) [email protected] domainkeys=fail (message has been altered) [email protected]

My guess is, that postfix changes/adds some headers.
But to get real confused. We are also sending our emails over this setup and depending on the receiver the DKIM is correct. These emails are generated by php and mail library.

Authentication-Results:; domainkeys=pass (ok);; dkim=pass (ok)
AOL fails:

X-AOL-SCOLL-AUTHENTICATION: mail_rly_antispam_dkim-d014.1 ; domain : DKIM : fail

the most Postfix settings are default

# dkimproxy content_filter= receive_override_options = no_unknown_recipient_checks,no_header_body_checks

How can I debug this to find out where the header changes happens?