Subdomains' user registration : Activation mail not sent

Hi there,

I have multisite with those plugins installed:

- Multisite Privacy
- Join My Multisite
- WP Approve User
- Custom New User Notifications Fk ("personnal" plugin that add mail notification to subsites' admin when somebody register on his site)

This works well, but activating Sites Pro blocks activation mail on subdomains' user registration.

I tried to deactivate one by one the plugins, and Pro sites seems to be responsible for that.

You can reproduce the issue here:

Any idea to fix that please ?

Grant Access active.