Hello @de-groene-vallei,
The customizer is not working due to various reason . It could be due to the plugin conflict . So please try disabling the plugin one by one and verify the issue . If the issue is solved you will know the exact plugin that is creating the conflict .
Also, verify whether you are having different WordPress Address (URL) and Site Address (URL) or not . Your WordPress Address (URL) and Site Address (URL) has to be identical. The preview will not load if you are having a different URL.
Hope this helps .
Kindly let us know if above steps does not solve your issue .
Best Regards!!