Q:  What is the meaning of this error message:  "The asset is Metadata Restricted and visibility flags cannot be updated"?  It sometimes appears near the top of the entry when opening an asset.

A:  This message appears when the Access Rights and License (Asset level) > Choose access rights policy is set to Metadata Restricted.

Metadata Restricted is an old setting which predates the ability to customize the "Display in portal" and "Display in profiles" checkboxes on the asset. 

You may want to remove it (set to blank) and replace it with more specific settings for the asset using the 2 display checkboxes.