The error has been posted on the forums a few times, with a fix related to a material and another fix related to merging meshes.
The error seems to say that there is a mesh with no name which isn’t a part of the hierarchy.
Could this be related to blend shapes? FBX settings are set to use input connections.
This specific error doesn’t seem to be in the documentation, is it documented anywhere else?
I suspected it could be related to geometry that is only included in the rig through a Maya point constraint, but that doesn’t seem to be the case with this rig.
Tried FBX 2018 and 2020 using Maya 2022.2 Game Exporter