Cups not Printing

Problem 'Permission Denied'

After upgrading it happens that cups is unable to print and reports:

Unable to open print file: Permission denied

Solution

It has something to do wit the cups configuration.
Therefor delete the /etc/cups directory content or move it to a backup directory at /root/cups-backup just in case.

Finaly reinstall some packages so the configuration files are recreated.

apt install --reinstall cups-daemon printer-driver-cups-pdf

Maybe a restart of some daemons is needed like.

systemctl restart cups cups-browsed

Problem (cups-pki-invalid)

To solve this delete the SSL certificate in /etc/cups/ssl for the printer or all of them.