Thanks for reporting as @pkiller mentioned this should be fixed if you create the system folder. I have also fixed it on master as well as the error messages so you should get a better error message if the upload fails.
These projects look really solid - they are drop-in replacements for Redis and should offer the most closely matched performance without the need to keep everything in RAM:
For instance, if I've set up a private group in my forum that has view and posting privileges in a hidden category, is there a way to allow only the posts from those group members, that were posted in that hidden category, showing up in the group feed?
This would be something we'd be interested in adding to NodeBB core... maybe a group setting to define which categories to show posts in the list.