Dear Chris,
The blinking SLIP REVERSE led indicates that DDJ-RR is not getting enough power. It might be a defect or a sign that there is not enough power coming in. I presume you are using it with USB cable only?
In that case, please connect the AC adapter as well. Or try using another USB output, one that can surely deliver at least 1A. (Don't use USB hubs).
I don't think it has anything to do with the rekordbox 4.2.1 update, this might be coincidence that this happened at the same time.