Hi @janw.oostendorp,
well the first thing I would do is check what your UEFI settings are, do you have "Secure Boot" disabled?, are you using "Legacy Boot".
In the pic above and where it states "OS to boot by default", did it have any other options that you could select?. 
If you scroll down to section 4 of the partition guide, you will find some info about UEFI.
If you have all your data backed up, you might consider using the "Something else method" to do a fresh install?:
When you get to this window, be careful where you put the boot-loader and also watch what partitions you format/use:
