"Fix Mask" appears in the Animation importer after importing motion file.
Using the "Fix Mask" button fixes the problem, but the same file imports fine (no Mask issue) in 5.2.4

repro:
-import attached Basicmoves.unitypackage
-select any file
-open the Importer Animation tab
-expand the Mask option...
result: "Mask does not match hierarchy. Animation might not import correctly"
"Fix Mask" button appears.

this issue is repro in...
Version 5.5.0a1 (4fc44dd1ef83)
Tue, 14 Jun 2016 21:23:31 GMT
Branch: 5.5/animation/dev
... and ...
Version 5.4.0b22 (518a15457bf6)
Fri, 10 Jun 2016 20:29:39 GMT
Branch: 5.4/staging
... and ...
Version 5.3.5p4 (ca3be209c65a)
Sat, 11 Jun 2016 20:22:01 GMT
Branch: 5.3/patch-release

Not repro in...
Version 5.2.4f1 (98095704e6fe)
Fri, 11 Dec 2015 22:00:30 GMT
Branch: 5.2/release