mistle ah yeah it won't work because the
Compat extender no longer exists.
This should definitely be logged somewhere. Either the Flarum log file or the webserver global log file.
It would have to be replaced with a service provider, or a custom extender.
I don't remember if this can be added to
config.php directly. It might only be the cookie configuration that can be put in
config.php and not the session stuff.