If there is no picture both in Android and Libreelec I would assume a hardware issue. Check if the HDMI socket is properly soldered.
Yeah, as I said, this was my first thought, as well, BUT there
are really a lot of guys posting this on several YT videos and
forum posts (even here some pages back) and the only solution
up to now was to use 720p TV and then set it up there, but
who has this stuff, anyway?
I still have a very old TV set, but, of course, it has no HDMI. :\
I thought, I could overcome this within Libreelec setting-wise
then.
Btw, is the builds' creator on holiday or something like that?
Maybe he has an idea/solution.