ok, strange.
You can also try a text locator like
Click "Zoeken"
I guess you have already tried xpaths like
Click //button[@type='submit']
etc.
If that all does not work, you could check in the Developer Tools if the button is inside an iframe element for whatever reason. Then you can use the iframe syntax of the Browser Library using >>>
I often go into Debug mode and try out my locators using the Highlight Elements Keyword, when having problems to find elements