Once again. My solution is not a good one 
Some system logs (auth.log for example) did not work any more. I decided to start syslog-ng with default service file via sysvinit, not via systemd:
sudo mv /lib/systemd/system/syslog-ng.service /lib/systemd/system/syslog-ng.service.bak
sudo systemctl daemon-reload
sudo /etc/init.d/syslog-ng start
In my owncloud.cfg I configured source(s_src), in syslog-ng.conf I deleted source(s_dgram).