I will double check, but if I am running 7.3 I am expecting much of my own tools and code to break. Before I saw your reply I checked restoring a backup to a dev environment and that all works fine. So it does indeed seem to be file or server related. I'll schedule a restart for Monday or so and...
When I try to enter my admin panel I get shown an error "An unexpected error occurred. Please try again later." If I look in the database what the last error is that is thrown I see:
message: The session key must be overridden.
The stack trace:
#0 src/XF/Container.php(28)...