The belcotax application of the government is apparently not yet ready to work with the latest version of Java. The way to solve this problem is (more info can also be found on oracle's website):

Record configuration screen of java. There are two ways to do this:

Way 1:           
  • Click on your Windows start button
  • At 'search programs and files' type in: Configure java
  • In the list above you will be able to click on 'Configure java'.

Way 2: 

Click on the Windows start button, then click on 'Control Panel' and then on 'Java'.
  • Click on the tab Security
  • Click on the 'Edit Site List' button
  • Click on the 'Add' button
  • In the list 'Location' please fill in the URL of belcotax:
  • Click on the 'OK' button
  • Click on the 'OK' button once more
  • Close and restart your browser