s2member plugin disables user registration

I have a MU install of WordPress and want free (Lev 0) subscribers and further levels (1-4). Although I select allow Lev-0 subscriptions, the wp-signup.php URL still says “Registration has been disabled”.
Looking at the value in Network > Settings, I can see that the “Registration has been disabled” option is checked and greyed out (because it’s controlled by s2Member).

Disabling the s2Member plugin, changing the Network > Settings option to allow registration and then re-enabling the plugin doesn’t help. The setting reverts to “Registration has been disabled”.