/var/cache/debconf/config.dat is locked by another process: Resource

ERROR: /var/cache/debconf/config.dat is locked by another process: Resource temporarily unavailable

$ sudo fuser /var/cache/<wbr>debconf/<wbr>config.<wbr>dat<br> /var/cache/<wbr>debconf/<wbr>config.<wbr>dat: 4503</wbr></wbr></wbr></wbr></wbr></wbr>

$ ps aux | grep 4503
root 4503 0.0 0.8 10816 9088 ? S 08:39 0:00 /usr/bin/perl -w /usr/share/<wbr>debconf/<wbr>frontend /usr/sbin/<wbr>update-<wbr>grub</wbr></wbr></wbr></wbr>

$ sudo kill 4503

After killing the process (4503 in this case) and running update-manager again, the upgrade can be completed successfully.