I am a little bit confused by the different messages. The last message is about adding widgets to WordPress? That functionality is the normal functionality of the WordPress and if it is not working for you the cause should be something else.
You can test it by changing the theme from X to 2016 or another default WordPress theme and deactivate the Cornerstone plugin. If the problem persists then it is caused by another issue.
I suggest that you check the possible 3rd party plugin conflict by deactivating all plugins and check the case. If you can add widgets then, you need to activate the plugins one by one to find out the one which is causing the issue.
Also, I am not sure what section of the http://fietsverzekeringvergelijken.net/ site is not working.
We are here to help but we need to get detailed information on each case to be able to give you proper suggestion. I suggest that if you encounter a new issue you open up a separate topic so we can focus on each question. Even if you have the same issue on another website and the first question solution doesn't apply for the first one you need to open up a separate topic as the environmental factors may differ.
Finally, I suggest that you do not send questions on a topic before getting an answer as it will reset the queue and the topic goes to the end of the waiting list for an answer.
On each question kindly give us context, detailed information with screenshots so that we know what is the problem.
In general, always try to check these troubleshooting steps despite the symptoms as it proves to be effective:
Thank you for your cooperation and patience.