BuddyPress.org

It must be because you are running an older version of BP alunsina. Sign up goes to the registration form. What happens when you upgrade BP? White screen of death? There have been lots of changes recently including changes to the directory structure for the member themes. See: https://buddypress.org/forums/topic.php?id=1675

That was effective rev 1211. Make sure you update the member and home themes also. Lots of changes in the security area from wpmu 2.6 to 2.7.

When you upgrade to the latest bp what do the apache error logs say? That’s the only way to find out what is going wrong. White screen of death means that a fatal php error is happening.