We investigated the issue about the bone names prefixed with “[FONT=courier new]mixamorig”, causing the plugin to fail detecting the skeletons as being compatible with Mixamo. The issue was a bug in UE 4.20.0 and should have been fixed in UE 4.20.1. Please update your UE4 installation and verify if the plugin now works as expected.
Here some more details:
- Official issue: Unreal Engine Issues and Bug Tracker (UE-62067)
- Official fix: https://github.com/EpicGames/UnrealEngine/commit/94d60d42f96113c129575032cd92055d7c1c0326
- Can't import animation in 4.20 - Character & Animation - Unreal Engine Forums
- Skeletal mesh and joint name issue with 4.20 (Maya namespaces) - Character & Animation - Unreal Engine Forums
- word of WARNING for mixamo rigs - Character & Animation - Unreal Engine Forums