I have some workspaces that are quite 'old'. They have been used with several versions of eclipse. It happens quite often, when I add or remove a project that those workspaces do not build. Eclipse complains that some project should be build first and I should clean the workspace. Well, cleaning the workspace does not help. So, I opened and closed some projects until I got it to work. Very frustrating! It seems that the build order is wrong.
Today, I had the problem again. But I looked at Preferences->General->Workspace->"Build Order". There nothing was set and "use default build order" was unchecked. I checked use default build order and now it works!
Am I the only one having this problem? I wonder if something has changed between two versions of eclipse and the "Build Order" setting did not migrate well and caused that problem.