SOLVED
The issue was apparently caused by adding an endpoint through the endpoint manager which resulted in a change in port settings. Both the SIP and PJSIP ports were put on 5060, which is not possible. We have manually changed the ports of the PJSIP to 5062 – 5063, and everything was working again, immediately.
=> Thanks to the help of Christian Riegert of Christian Riegert IT Service
