I had a project where we got a similar error. It seems to have to do with the amount of bones / virtual bones on a skeleton. Removing all virtual bones from the target skeleton allowed us to assign skeletal meshes to another skeleton / import to the skeleton properly. Sometimes retargetting animations caused a similar error and we fixed it by adding a single virtual bone to the root bone of the target skeleton (the one receiving the animations) and it then allowed us to retarget.
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