I changed buffer from 100 to 0, now it shows the 'Reference system layer'. But when I make a selection in field 'GEOCODE_NAAM', (for example 'Alkmaar') the map zooms in, but it stop showing the 'Reference system layer':
This is the result:
When I click on a zoom button (+ or -) the layer pops up again:(the purple and blue lines)
Ok, we found the problem, it's the wms service at prorail.nl that complains that the wms image is to big.
If you make the map window smaller it works, that's why the "margin" parameter had some effect.
To spot the error, opne up the dev console ctrl shift right mouse click -> "Show dev tools" and check network tab for the image that fails. Open the image url in a browser:
<ServiceExceptionReport xmlns="http://www.opengis.net/ogc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1.3.0" xsi:schemaLocation="http://www.opengis.net/ogc http://schemas.opengis.net/wms/1.3.0/exceptions_1_3_0.xsd"><ServiceException code="InvalidFormat">Parameter 'width' contains unacceptable value.</ServiceException>