You have set a milling depth on the opening setup page?
Check on the machine setup that you have milling parameters set up there as well (XY feed. Spindle rpm etc)?
I happened to still have Rev 7.7.0 still available, so tried a milling outline on an old completed project - no problems there.
One further thought, you have a dimensioned board outline and some component on the schematic and board layout - I don't know what would happen if these are omitted.
I now use Rev 9.6.0 - never had a problem with this - and it has many more magic features!!
