So we installed the latest version. Now a different error pops up. I have attached a screenshot, it's in Dutch, but i'll translate it below. What could this be?
Your computer is unprotected
The kernel extension which is required for the Firewall, is not loaded because of an error. Try to restart macOS or reinstall the product.
Of course the Mac has been restarted and also the software has been reinstalled. But this doesn't seem to help.