The proper permissions for /tmp are root:root 1777 (drwxrwxrwt). Inadvertently setting it to 755 will cause X not to start as a normal user (among other problems).
My problem was caused by untar-ing a data.tar.gz from a Debian package directly in /tmp. The tarball contains an entry for "." thus it changed the permissions on /tmp when untarred.
Above: the culprit
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