When using a format where you specify the number of sub directories to use and/or the mode, the installation process complains that the directory is invalid.
Removing the N; and MODE; fixes the problem and re-enabling after the initial check seems to cause no side effects thus far.
session.save_path = "N;MODE;/path"
session.save_path = "3;/tmp/session"
session.save_path = "9;600;/tmp"