Fixed. Took me way too long to figure it out. I needed to set the permissions for the root session directory /var/opt/remi/php71/lib/php/session to be writeable by webserver:DomainUsers since I use PHP_LDAP. The command

chmod webserver:DomainUsers /var/opt/remi/php71/lib/php/session

did the trick.

Sorry, I figured it out. Looks like I was confusing files and folders. To hide files beginning with certain characters, you must use the configuration line:

$config['app']['hidden_file_names'][] = "~$*"