syslog-ng 3.7.3 was released yesterday. It is now packaged at https://build.opensuse.org/project/show/home:czanik:syslog-ng37 including a fixed syslog-ng.service file. Note: this is not the official openSUSE syslog-ng package (has a few more features enabled). Tomorrow I'll try to figure out, how to create an update package for Leap 42.1