I've hunted for reasons for this but nothing simlilar is popping up. I've created a simple walking character using parenting & classic tweens. There are only instances are of the same objects for each item/limb (one common error I found when searching was multiple objects created accross 1 limb, if you like!). Exporting as asset appears to work find, but in trying to then import that to another stage or to the library I get this error. No further details. Any 'common mistakes when exporting assets' I can run past?