I have the same problem on Bluehost.
Tried uninstalling all extensions, also back to default theme with all others deactivated - php7.0 would not work. Tried replacing all files and still not working. Other open source gallery in subfolder is fine though.
Manually updated .htaccess file - line for php handler to php70 when I changed the php.ini file.
Database server: MySQL 5.6.41-84.1
PHP version: 5.6.37 (cannot get phpbb3.2.5 to work using php 7.0 on bluehost - they say it is correct at their end)
Board version: 3.2.5
Just saw this (Apr 6).... will try and see if this does it - works;
mysql is not supported in PHP7 you need to change your config.php file to use mysqli, and purge the cache.