It's an infrequent and usually non-repeatable bug in SUp that Mirror.rb seems to encounter now and again. Some set ups and systems seem more prone than others. I developed this version of Mirror and the BugSplat comes when you mirror collections of certain types of geometry that aren't grouped - can't see a common denominator AND I've never had one***. You can try grouping before mirroring and exploding afterwards and see it that helps. It seems to be something to do with the way/order that SUp processes grouped objects - Mirror.rb temporarily groups your selection, makes the copy and mirrors it and then if desired deletes the original - it's often this deletion can lead to these rare splats.
There is a recent discovered clash between the Demeter.rb script and Mirror.rb (and Grow.rb) because Demeter has an aider-plugin called blankslate.rb and that seems to rewrite a bit of the way SUp+Ruby processes groups. This causes a crash every mirror+delete-original so it's unlikely to be your cause since it's intermittent ? ***I HAVE experienced that !
Until someone in SUp+Ruby fixes the flaky interpreter and group manipulation coding then occasional splats might randomly occur. So save beforehand and try pre-grouping what you are about to mirror - please let me know if you get any joy...