I know now what caused the bug: we decided to go smoothly with the etch 2 lenny transition and issued a 'apt-get upgrade' only, so we could take our time on the 'dist-upgrade', fearing for larger issues. However, only the 'dist-upgrade' pulled in the new courier-auth* packages which fixed the problems. I guess, the dependencies were not set properly, but the fix is simple: issue a 'apt-get dist-upgrade'.
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