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Opening a workspace with several projects, I was presented with the following error for two of the projects:
To resolve this error in eclipse do the following steps.

The resolution was to force a resave of the selected projects (and their .classpath files):

Open the project properties
Select Java Build Path > Libraries
Add a new, arbitrary library (to be deleted later) > OK
Wait for the workspace to refresh (or force a refresh of the project)
The error(s) will go away
Remove the dummy library
The only other references I could find were to make minor alterations of contents of the .classpath file.