Thank you for your reply. I have emptied the var/cache folder, without any luck. The page source reveals nothing as there is apparently nothing loading. The page is empty. Very strange.
We have control of the server, and was just doing a preperation to upgrade cs-cart to 4.3.6, so we asked a system administrator do update our system to the latest versions of php, apache, mysql etc...
config.local.php:
// Cache backend
// Available backends: file, sqlite, database, redis, xcache, apc
// To use sqlite cache the "sqlite3" PHP module should be installed
// To use xcache cache the "xcache" PHP module should be installed
// To use apc cache the "apc" PHP module should be installed
$config['cache_backend'] = 'file';
$config['cache_redis_server'] = 'localhost';
$config['cache_redis_global_ttl'] = 0; // set this if your cache size reaches Redis server memory size