Exporting assets, then Importing error; 'one or more files were not...'

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?