Bug#320470: Cups does not print to a serial printer
When we upgraded from woody to sarge, we noticed that our serial printers (we
use a cyclades serial board) stopped working. Raising the debug level in
cupsd.conf from info to debug, we noticed in /var/log/cups/error_log that the
file /usr/lib/cups/backend/serial was missing.
To solve this issue, we created a symbolic link using the following commands:
ln -s ../backend-available/serial .
So after restarting cupsys everything started working normally.