It happened to me once with earlier versions of Oxwall.
some of the 8 tables used by newsfeed It stank, and uninstalling the plugin Newsfeed and reinstalling it was solved, but all content is lost.
This time I could not reinstall it, fails "Invalid plugin key -'_Invalid_' provided" and then does not work more than the plugin administration panel, but not displayed.
Any ideas? My network with 5,000 members is out of walls.