A bit weired, but yesterday i messed around with a new multiplayer build and suddenly the interface stopped showing up. Now after coupel of trials and errors i removed the branch from the Possessed event (as suggested in your video), and voila interface works just fine. And more oddly afterwards i added a print string on that branch, it continued to work (with or without print). Really no idea what the issue was.
On a similar note, if you want to extend JCItemDataStruct, add new item, hit save on that screen. But then immediately quit-close the project (without saving), otherwise several components will break. Reopen and new added struct item is usable.