Thank you so much for all the effort!
tl;dr: Please do not make any further efforts for the time being.
I did some more testing and different settings by myself, too. After many many hours and reboots it looks like I could have found something. It looks like it could be a little wobbly contact in one of the power outlets. Beside different air flow scenarios I was trying four different power supply units. Two of them are new and especially suggested for the Pi by the berry shop. After a while I was annoyed by always getting them out of the socket and replacing them with another one, so I placed one (by coincidence an old pixel-phone power supply) in a different socket a little bit further away and this time it went stable for 7 hours up to now.
So the next thing I do is checking the next few days if this was just a coincidence today or if it really works this way...
...and if the problem comes back again, I get the kodi.old.log and come back here.
Meanwhile thank you very much again for all your investigation and please also send a heartfelt thank you to the dev.
All the best,
libelle