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