I can't add a Site from Network Dashboard after enabling Pro Sites

Hello team,

after activating Pro Sites I see an error page whenever I add a Site on my Network Dashboard.

I see:

"Are you sure you want to do this?

Please try again."

The "Please try again." is a link to wp-admin/network/site-new.php (the Add Site form).

Despite the error, the Site is created. Just concerned with the error. It's not generating logs (even with debug enabled) but I do not expect to see an error on the main plugin that will sustain my MultiSite websites creations.

Also, this is not intuitive I see "Please try again" and return to the same form (other people working with me may forget this is a weird bug and then just add, add, and add Sites continuously for not seeing they were actually added on every submit!).

This is happening on WP 4.1, using either pt_BR (Brazilian Portuguese) or default (English) language. I've tried disabling all plugins (Site creation worked fine), then I enabled just a few (5-6) and after enabling Pro Sites again, the error returned.