The “ugly” error message is due to an unhandled linux-specific situation in an underlying software library that the AxiDraw software uses. (We have submitted a patch to fix their error message, but it hasn’t been accepted yet.)
What the message *should* say instead of that is something like “Failed to connect to AxiDraw.” Since you’re seeing a difference versus fully unconnected, I’d recommend resetting the AxiDraw and/or restarting your computer. Unplug it from both power and USB, and then connect it by USB alone until you’re making a good connection again. If that doesn’t help, unplug it again and try restarting your computer before plugging in USB again.