Since I've installed a new front cam flex, i disconnected this one and started the iPhone again. When I did this it didn't turn off/ restart anymore. Seems like the flex i received was a faulty one, I ordered a few new ones and tried again, no more restarting anymore. So it seems to be the case that there is a lot of difference in quality regarding this front cam flexes!
The solution to this is very simple. You just have to heat the part extremely well (not to hot not to cold) and besides that, you need to pick the little black piece of tape AND the front cam flex in 1 go! If you try to separate them the chance is 75% you will damage the front cam flex and also have to replace this one.