Other SP peers seemed to have the same problem and many could solve it by installing a WCF hotfix (KB976462), however this did not work for me. I will show you three checks I did in order to have stuff working. 1. Check if the Business Data Connectivity Service is running. You can find it by going to "Central Administration" -> "Manage services on server". If it has stopped, start it again.
2. Make sure you've got a Business Data Connectivity Service Application set up in "Application Management" ->"Service Applications"->"Manage service applications"
3. The next one is what I overlooked and made me waste hours of my life. Go to "Application Management"->"Service Applications"->"Configure service applications associations". In here you need to check whether your BCS Application Proxy is checked, otherwise it won't work.