Multisite registration not allowed

Hi,

I’m running a multisite, and in my network settings the second radio button is selected (only account registration).
When I activate S2 member on my network, the radio button is disable and I can’t change it at all.
When I deactivate S2 member, network settings are enabled.

In S2 member configuration (multisite config panel and general plugin panel) I’m allowing open registration.
Any idea what I’m doing wrong ?

Thanks for your help,
Jennifer