Sorry, I mixed up the Scala versions with the IDE versions. I uninstalled Scala IDE 4.5.0 (that uses Scala 2.11) and installed version 4.6.1 (that uses Scala 2.12).
I would not normally add scala-compiler.jar but Googling the error message I found some posts that suggested this is a Scala compiler bug that needed this as a workaround. As said it did not work for me.
Back to Scala IDE 4.5 en Scala 2.11 for now until this is resolved.
Cheers,
Silvio