Humanoid Avatar with Additional Bones Not Working

Hi, I have a character with a rig that has additional bones for props it uses in the game. While importing I set up a humanoid avatar (making sure the "Keep Additional Bones" option was checked) leaving any aditional bone unassigned. Every animation was working fine when the last update of Unity came along.

Although the character still worked perfectly with additional bones animated, I had to make some changes to the rig, and now mecanim won't work with additional bones. Everytime I try to reimport either the first rig or the new one and try to configure the avatar, Unity doesn't create it (i.e. there's an "X" next to the "Configure..." button). The weird thing is that the previous avatar configured before the update still works fine, but if I try to create it again in another project or create another avatar with additional bones it just won't do it.

Any idea why it's not working anymore or if anything has to be changed/configured in a different way?

UPDATE 1
Delving further into the problem, it seems that whenever I attempt to manually configure the armature the avatar is not created. 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.