Okay, looks like some kind of trouble with SELinux booleans used by Plesk.
Try:
# getsebool -a | grep httpd_can_bind_all_ports
If it shows that it's off, then try to re-enable nginx as a reverse proxy server:
# /usr/local/psa/admin/bin/nginxmng -d
# /usr/local/psa/admin/bin/nginxmng -e
If there is no output at all (edit: from the getsebool command above), then this most probably means that you have wrong 'plesk' SELinux module version (or none at all), hence in much more trouble than only httpd start problems.
Hope this helps.