Thanks for reopening the thread.
I tried with the Estuary skin and the problem persisted. Still, i've found a solution.
Libreelec/Kodi set the resolution autimatically, at least as far as i can recall, to 1920x1080p@60 fps. I changed it to 50 fps and the resolution became "DESKTOP" but it seems to be 1080p still. After a reboot all issues were gone. I can't wrap my head around why it made a difference but maybe somebody else can.