After a week of trial and crashing I finally found a way to render my videos without crashing.
The project I am working on has a mixed bag of input : some HD some UHD. The HD came mostly from Panasonic FZ1000 cameras, who only have 8 bit color depth, so I left pixel format at 8 bit full range.
After changing that to 32 bit floating point video level, suddenly everything rendered without problems...
Although I have no idea why this works with the input from the older camera's, and also why some selections did render, and why everything rendered without colorgrading when pixel format was at 8 bit full range.
As many here are having problems with rendering, I wanted to share this info.