When you have an ARA extension open in Cubase you can’t open the sample editor because the ARA extension will be the actual editor. This goes for all ARA extensions, not only SpectraLayers. You’ll have to leave the ARA extension through “Make extension permanent” or “Remove extension from selected event” to be able to open the Sample editor in Cubase.
This is the way the host (Cubase, Nuendo) has implemented the ARA protocol and I don’t think it is an issue with the ARA extension/SpectraLayers per se.
Try to ask at the Cubase forum, maybe someone there could help, e.g. if there are any settings to change in Cubase (I’m not aware of any though).