I fixed it myself.
In the C:\ProgramData\Qlik\Sense folder, there was no host.cfg file. No idea why this wasn't created by the installation of Qlik Sense itself.
1. write down the full FQDN name of the server
2. Go to Base64 Encode and Decode - Online, and paste this FQDN name and press the Encode button
3. Copy the encoded result to a new file named host.cfg and save this in the C:\ProgramData\Qlik\Sense folder
4. Open a command prompt
5. Stop all Qlik services
6. go to folder "C:\Program Files\Qlik\Sense\Repository"
7. Type "Repository.exe -bootstrap -standalone -restorehostname". See also 'restore a qlik website'. Start the Qlik Sense service dispatched service. If all OK, the system should end with 'Bootstrap mode has terminated. Press ENTER to exit..'.
8. Start all other Qlik services
This should fix it. If not, probably you first have to delete the certificates and go back to step 7 to recreate them.
Good luck
