Brand new WP MU 3.5 problems: registration emails and PHP Warnings


I just made a brand new installation of WP 3.5, enabled MU and installed some WPMU DEV plugins.

Then I tried to create a test user using the form on the website. The registration Email never arrived.

After that, I tried to create an user using the Network Panel. I get this errors:

Warning: sprintf() [function.sprintf]: Too few arguments in /home/casalmai/public_html/wp-includes/ms-functions.php on line 1065

Warning: Cannot modify header information - headers already sent by (output started at /home/casalmai/public_html/wp-includes/ms-functions.php:1065) in /home/casalmai/public_html/wp-includes/pluggable.php on line 876

And the Email with the password never arrived too!

I tried to repeat all the above steps with all plugins disable and I get the same thing.

Do you guys have any idea about this?

I'm not a noob on WP but I'm not a programmer either. I really don't know how to debug this.

Must say again, it's a brand new installation and only used plugins from WPMU DEV.

Thanks in advance!