Is RMX 1.1.13 supposed to work in macOS 10.13.6? I cannot get Scaler to scale anything in any way. Instead I get the message “Not all masters in all glyphs could be scaled.”.
Why do you think the version of MacOS is causing this? It you get to a point where you get a meaningful error, your version of MacOS is OK. But what version of Glyphs do you have.
And the outlines are fully compatible?
G3  and yes, the outlines are compatible.
::edit:: Problem solved; it was one of my settings.