I stumbled on this when working on bug 10433.

Trying to start syslog-ng I just get an error:
root@qemux86:~# /etc/init.d/syslog start
Starting syslog-ng:[2016-11-21T14:49:48.324805] WARNING: Configuration file format is too old, syslog-ng is running in compatibility mode Please update it to use the syslog-ng 3.6 format at your time of convinience, compatibility mode can operate less efficiently in some cases. To upgrade the configuration, please review the warnings about incompatible changes printed by syslog-ng, and once completed change the @version header at the top of the configuration file.;
[2016-11-21T14:49:48.612669] Error performing backtick substitution in configuration file; error='missing closing backtick (`) character', filename='/etc/syslog-ng/syslog-ng.conf', line='55:1'
[2016-11-21T14:49:48.613652] Fatal error in configuration lexer, giving up; error='backtick substitution failure'
Error parsing config, syntax error, unexpected LL_ERROR, expecting $end in /etc/syslog-ng/syslog-ng.conf at line 55, column 1:

# Some `catch-all' logfiles.
^

syslog-ng documentation: http://www.balabit.com/support/documentation/?product=syslog-ng
mailing list: https://lists.balabit.hu/mailman/listinfo/syslog-ng