Same issue here. Setting up a new site, it worked fine a couple of days ago, went to access and it's broken.
Fatal error: Call to undefined function get_front_page_template() in /home/mainaccount/newsite/wp-includes/template-loader.php on line 26
When I try to access the dashboard, I get this:
Database Update Required
WordPress has been updated! Before we send you on your way, we have to update your database to the newest version.
The update process may take a little while, so please be patient.
"Update WordPress Database" button
If I try to update the database, I get this error:
Fatal error: Class 'WP_Theme' not found in /home/mainaccount/newsite/wp-includes/theme.php on line 106