Hello Michiel van Oosterhout,
Did you, by any chance, try removing the Newsletters module? Such and error might occur because NewslettersResources are referenced in the Basic settings sidebar template, so removing the module gives you this error when accessing Administration >> Settings.
You need to register it again now, and restart the application. You do that by putting the following line in your AppData/Sitefinity/Configuration/SystemConfig.config file of your SitefinityWebApp project
Restart the application and the Newsletters module should be installed, and you will be able to access the Settings page.
the Telerik team