Manual Configuration for Avatars Broken?

After I updated Unity to version 4.2.2 whenever I try to manually configure an avatar for a character, after making sure the character is in T-pose, and every bone allocation on the mapping inspector is green, Unity won't create the avatar (i.e. there's an "X" next to the "Configure..." button).

Even on a fully functional avatar with no additional bones, if I unassign for example a finger bone, apply changes, and then reasign it to the same position as Unity did automatically (wich did succesfully create the avatar) it won't accept it as a valid configuration.

Any manual configuration results in the avatar not being created. The weird thing is that I had succesfully done an avatar this way before the update, but although I repeat the process with the same files I can't create the avatar again.

Am I doing something wrong? If not, how do I fix it?