Hi there, it looks like the iframe containing the Twitter timeline widget is not appearing in the source code within the page on your site at http://www.lefsvj.nl. I tried entering your widget ID into a test site and can see the Tweets appearing for your account @LeFsvj.
I suggest to compare the plugins or hosting environment between your two sites. Is it possible that a security plugin or hosting account setting on lefsvj.nl is blocking embedded iframe content from being displayed?
You might test this by embedding a google map code into a text widget, to see if the iframe loads -- I've saved an example you can try here: