Hi!
I've been using Libreelec installed on a Zotac ZBOX-CI660NANO (specs: https://www.zotac.com/us/product/mini_pcs/ci660-nano#spec) with an old Samsung smart TV without any issues but only with 1280x720 resolution. It defaulted to that and I didn't notice at the time altough it was a FHD TV.
I changed the TV to a Hisense H40B5600. When I plugged in it defaulted to 1024x768 resolution and it couldn't be set higher. After some time I figured out that the problem was that the aspect ratio on the TV was set to automatic and somehow it defaulted to 4:3 with Kodi. When I forced 16:9 on the TV it allowed me to change the resolution. There is two option for 1080p and there is 1080i as well. I changed the resolution to 1080i since that is what my TV defaults to when I use a set-top box from the TV provider (1080i @ 50Hz).
First it seemed that it works but when I started a movie and wanted to go back to the menu the HDMI disconnected and it didn't connect until I restarted the miniPC. After trying again the same issue arised. Then I tried to change the resolutions and framerate (from 60 to 50) and nothing helped. Every time I changed it, the hdmi cable had to be removed and plugged in again to display anything. (Note that the TV didn't say the HDMI was removed, it's just that there was no signal. When I unplugged it, it always had the message, that it was unplugged). I bought a brand new high speed HDMI cable but the issue remained. I was trying the different resolutions but when I went back to 1080i@50Hz it stuck and there were no signal ever again. I can't change the resolution on the HTML page either. When I try and press Save the display never comes back and once I restart it, it goes back to 1080i@50Hz. Libreelec is online and everything seems to work, it's just that there is no HDMI signal. The miniPC worked before providing 1080p/i content and the TV also works with my computer and the set-top box displaying 1080p. When I restart the miniPC it displays the Zotac logo then the LibreELEC logo and when it loads, the signal is gone.
Tl;dr: Once Libreelec loads, the TV loses the HDMI signal.
Here's the log after restarting the miniPC:
http://ix.io/4Jqw