I got the following error message:
W: http://u58733p55594.web0093.zxcs-klant.nl/repo/dists/unstable/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
To fix this problem, do the following:
sudo apt-key export D7A223ABAA7CE436 | sudo gpg --dearmour -o /etc/apt/trusted.gpg.d/mintppc.gpg