This looks like a problem with the apparmour config for cups, I suggest if you add /dev/ttyUSB* to /etc/apparmor.d/usr.sbin.cupsd in the same way as the /dev/ttyS* rw, line it should fix it.
Dave
This looks like a problem with the apparmour config for cups, I suggest if you add /dev/ttyUSB* to /etc/apparmor. d/usr.sbin. cupsd
in the same way as the /dev/ttyS* rw, line it should fix it.
Dave