sudo fuser -v /var/cache/debconf/config.dat returned nothing.
So, I made a backup of the files contained in /var/cache/debconf/ and have deleted all the original files in the same directory. After that, I've "restored" the backup and the upgrade worked fine.
Complementing #8 and #11:
sudo fuser -v /var/cache/ debconf/ config. dat returned nothing.
So, I made a backup of the files contained in /var/cache/debconf/ and have deleted all the original files in the same directory. After that, I've "restored" the backup and the upgrade worked fine.