[mamachan]

 This is very weird.

Currently I'm playing my 4k HDR files directly from my Qnap NAS using SPMC 16.7.3 without a single problem. In fact, Plex is the one giving me problems swith a lot of "buffering" screens. On the other hand, SPMC works perfectly...  

Can you post your audio/video settings please?
SPMC and MI BOX.
Many thanks  

I just realized what your problem is, mate.

I'm almost convinced that you won't have the advacedsettings.xml file on your kodi/SPMC, which greatly improves transfer rates...

Take a look, put this advancedsettings.xml file, restart the system and try to play any of the 4K HDR files again  

I do not have the advancedsettings.xml file neither on kodi or on spmc.
But on Kodi Kripton 4K files works fine, on SPMC no.

Where i can found advancedsettings.xml file? i'd like to tray it.
Thanks