Great question — answering as the UphillTech team:
You can try restoring the default udev.conf . After mv /etc/udev/udev.conf /etc/udev/udev.conf.broken (in case the file actually exists but is unreadable for some other reason e.g. a symlink to nowhere) apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall udev (Apparently for some packages it's necessary to prefix with UCF_FORCE_CONFFMISS=1 instead, joy. But I don't think the udev package uses ucf). However if this is more than an accident affecting the one file, you're probably best making backup copies of anything you want to keep and re-installing the OS as a whole. You don't know what caused it; I can't think of anything obvious. If you try fixing files one at a time,...
Full answer by sourcejedi: https://unix.stackexchange.com/a/271347 (CC BY-SA 4.0).
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