I've just discovered a ticket in the Tor bug tracker about the same issue. The user there had upgraded to 0.2.7.6-1~d80.jessie+1 when the problem started occurring.
"obfs4proxy cannot bind to <1024 port with systemd hardened service unit" https://bugs.torproject.org/18356 However, the workaround suggested there (setting NoNewPrivileges=no) did not work for me. I set NoNewPrivileges=no in both /lib/systemd/system/tor/[email protected] and /lib/systemd/system/tor/[email protected].
