What i forgot to mention at the time (and postings seem to not editable).
If that happens i can sometimes switch the hdmi input in the avr to some other port, wait a few seconds, swiotch back and the picture gets shown, other times i need to do a hard reset (probably a reboot would work too if i had some other control at hand which can cause that).
And here is a log.....no idea if it is any help at all, as that got created after a reboot (which i needed to do after a reboot as the Wireless connection was not active after the reboot when the detection problem happend..without debug log active) when that happend, and no idea if it internally always acts the same at that part (not even sure if it is completely a libreelec/kodi problem, as i do not know at which point at boot the hdmi detection kicks in)