I found the root problem and could fix it. It was a configuration problem (nearly) unrelated to netfilter-persistent. Due to netfilter-persistent's systemd file, modules have been loaded via a systemd dependency.
This caused /etc/modules being executed, which contained the line > snd_hda_intel power_save=1 which seems to be valid according to > modinfo snd_hda_intel but systemd failed anyway to execute this (with kernel 3.15) and and thus did abort to load netfilter-persistent. As netfilter-persistent is loaded in the postinst script, the configuration of the package failed. I will close this bug report and hope that the above information can help if someone has a similar problem.
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