Reproduction steps:
1) Open attached project, proceed to models folder and select riggedsamplecharacter2.fbx
2) Under rig tab in inspector change any setting back and forth.
3) Select animations tab in the inspector.
4) In the windows that pops up hit apply.

Expected result: Animations tab opens up / model loads up.
Actual result: Loading window appears and stays on "loading small assets".

Note: if you terminate unity and relaunch project, asset gets replaced by default asset (according to the editor console), thus in order to reproduce it again - project needs to be deleted and unpacked again.

Reproduced with: 5.3.2p2 5.4.0b5
Does not reproduce with: 5.2.4f1