If you open a Terminal and type "groups", does the resulting output include "uucp"? You might need to delete the contents of "/var/lock" (which you can get to from the Finder: "Go > Go To Folder...").
I've just resovled the problem, and it turns out there was a missing directory in /var/spools
The directory for uucp was missing, creating this and and chmodding it to 775 fixed the serial communication. Just thought others might give this a try if the other fixes don't work.