Comment 4 for bug 255200

I'd recommend getting an strace of the applications in question to figure out exactly which devices they're trying to access and what errors they're failing with.

From the point of view of a process in group dialout, 660 root:dialout and 666 root:dialout are precisely equivalent, so there is something more subtle happening here. Hitting it with 666 will no doubt resolve it for you but is not going to be the right answer for Ubuntu.