laravel 5 ->getRealPath() doenst show correct value

Chances are your account's root is /data/sites/web/christophvhbe, and that getRealPath is entirely accurate (what you see in FTP is likely chrooted). As a result, your $imagePath is actually the problem.

$imagePath = '/data/sites/web/christophvhbe/images/producten/'. $fileName;

Or, better yet, use Laravel's base_path and/or public_path helpers so if you ever change hosting it keeps working if the paths change:

$imagePath = public_path() . '/images/producten/' . $fileName;

If you consult your error logs, I'll bet you find permissions errors trying to create a file in the server's /images directory, which on shared hosting you definitely won't be able to create or access.