I got hit with this issue the other day after importing a project out of Team Foundation Server, but this 'can' happens when switching workspaces which is where I saw it before, but the fix is the same and is trivial. Simply clean your project.
Go to the menu Project -> Clean
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It has worked every time for me (once I remembered.) I am hoping taking time to write this down does the following:
1) helps someone else in their time of need.
2) helps me remember this simple task instead of me staring dim witted at the error message and throwing my arms up in another fit of nerd rage...
#2 is likely behavior that will remain unchanged. For the rest of you I hope this helps. ;)