The internal scion of eclipsefp has no cabal-dependencies command. It is invoked by eclipsefp in class CabalDependenciesCommand.
Eclipsefp and the version of the bundled scion seem not to match.
Which version are you using? I assume it's the development version? If you do get eclipsefp from github, you need to manually delete the .metadata/.plugins/net.sf.eclipsefp.haskell.scion.client/scion-0.1.0.4 directory to get it recreated with the proper code. When updating eclipsefp when the new version is out the scion version will be increased so user will get the new code automatically.
Thank you for your instant reply. Deleting the scion-0.1.0.4 directory helps.