[tymoxa]

So why doesn't Kodi play those discs? Even if I try .iso format or BDMV format. It just stands there and do nothing.
Is there anything that the end user can do to play those discs?

Because UHD is slightly different from BD. To play UHD in Kodi right now you can do a little hack. In HEX editor replace:
in index.bdmv: INDX0300 to INDX0200
in MovieObject.bdmv: MOBJ0300 to MOBJ0200
in *.mpls: MPLS0300 to MPLS0200
in *.clpi: HDMV0300 to HDMV0200

Basicaly that's it, you can play at least main title of UHD disc in Kodi after that.

But for BD-J - it is not enough because:
1) you need to replace BDJO0300 to BDJO0200 in all *.bdjo and BDID0300 to BDID0200 in id.bdmv
2) UHD players have more registers and java can check those. And if player it's not Profile 6 for example - disc won't play with menus.

I believe developers can add couple of lines of code to libbluray and this hack will be optional.