Hi Jeroen. I tried your map on my iphone, and can reproduce the problem of the white box. Checking in Firefox I don’t see any problems on desktop, but I I’m not aware of any debug tools on my phone browser. I’m not exactly sure why it would be happening, but have you tried checking for a plugin conflict? Turn off all non-SLP plugins, and revert to a standard WP theme, like twenty fourteen. Then see if the problem goes away. If this solves it, then it’s a plugin conflict.
If its a plugin/theme conflict, then turn each component back on one-by-one until you find the cause.