After microupdate MU #14 was installed for Parallels Plesk 12.0.18 static files (CSS/JS/JPG) are not loading on web sites.
Links for the vhosts configuration files are removed. Nginx stops serve vhosts. The issue is considered to be a software issue PPPM-1930 .
As a temporary workaround please run the following command to fix the issue:
# /usr/local/psa/admin/sbin/httpdmng --reconfigure-all
The issue has been fixed in microupdate MU #15 . Please install latest Plesk updates.