I got the following error trying to apply "Outline stroke" to a somewhat complex group on a small (3.33in) artboard.
The Operation cannot complete because of an unknown error. [%s]
Error: 0SEG
Well, thanks for that.
My work around was to first scale the group a bunch (after going to "Object > Transform > Scale..." and checking "Scale Strokes & Effects") and then apply "Outline Stroke".