page-not-found when activating user or site account

Hi to everyone, im using wp 3.6 with only Pro Sites plugin installed with sub-domain option.
The issue is when i click on the link at the email received to activate the user or site account it opens a new page displaying -> "account activation successfully " following with the username and password to login..., but in the browser title says Page Not Found.

I have also tried to activate the user account or site account before Pro Sites being installed, and it happens with multisite turned on only!

is this a wordpress 3.6 bug or server issue?

Thanks