The project cannot be built until the build path errors are resolved.
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.