was facing this issue and couldn’t find a proper place to look into then I got to know that this issue is surely related to the view..

After serious debugging found that this issue is mainly because of the usage of a taskflow as a region but the taskflow is not having a default activity..

The default activity somehow got removed and setting it back into the place made my day.

Attempt to validate an already invalid RegionSite:
  <_handleException> ADF_FACES-60098:Faces lifecycle receives unhandled exceptions in phase RENDER_RESPONSE 6
 java.lang.IllegalStateException: Attempt to validate an already invalid RegionSite: