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.