>>2859634If you mean videorelated, it's likely caused by bad modeling practices. Namely, not fixing the deformation order of IK bones after moving them above their chain in the list. While PMXE doesn't consider it an error and it works with MMD's IK solver, it is definitely an oversight. Although that can also happen with, say, a mocap motion that combines FK and IK retargeting (like the ones made by mobiusP).
Ademar claims Blender is much easier to animate in, but I don't know.
>>2859640Have you tried googling the link before asking? (Assuming your government doesn't censor VK links.)
(For reference, the Nico number of the associated distribution video/image is the most common Japanese password.)