Why is the rotation missing when I animate?

Hi, Unity animation system uses objects names to know what property is animated. That is, the full 'path' from the GameObject that has the Animator component (like character/torso/shoulder/arm/hand). If you rename one of those objects, the animation system won't be able to find the property to modify. It can also happen when renaming objects or changing parenting in an external animation software (Maya, Blender...).

If you know exactly what you changed/renamed and you are a little technical, it's still possible to rename objects inside the animation file itself (the .anim file) as it's a text file. You'll find properties like "m_Name" or "Path:" that you can edit. It can be a little tricky so always make a backup copy before modifying though :-)

EDIT: it is also possible to rename a object in the animation tab (select a property and hit "F2", or a long double click)