After turning my application running under SpringSource Tool Suite into a web app, then trying to run pre-existing junit tests, I started getting the error:
Plug-in org.maven.ide.eclipse.jdt was unable to load class org.maven.ide.eclipse.jdt.internal.launch.MavenRuntimeClasspathProvider.
An error occurred while automatically activating bundle org.maven.ide.eclipse.jdt (810).
The fix was to delete all the existing Run configurations (Run As / Run Configurations… / select m2 custom Run Configurations and delete them).
Restart STS (may not be necessary), and it now works!

