Any ideas on this one? I have two Drupal 8 sites running on distinct virtual hosts in Apache2 on an Ubuntu 14.04.5 LTS linode. One site been running fine for a few months, the other has just been installed (with a distinct document root, separate MYSQL database). This is not a multisite, shared-code setup. Site 1 is at 8.1.7, Site 2 at 8.1.8
The virtual host configuration works - I've tested it with placeholding index.php files.