Yes i tend to agree that this device is getting shafted by Garmin's server for some weird reason. The message "Cannot authenticate maps" is related to a missing or inappropriate GMA file. A problem with unlocking is what invokes the "Cannot unlock maps" message. Problem is that we can parse an UNL file with [Only registered and activated users can see links. ] and even create an unlock code with JetMouse for a detail map to be used on a specific device without GMA in its firmware, but we're unable to read, replicate or spoof GMA files. You can use InfoUC to easily check that the UNL code they've now supplied is for the right map (FID and Product) and for your nuvi 58 (UID). (*See edit below*).

I think it's a fair question to ask Garmin support why the heck they're supplying your device with invalid files. The service desk drones likely won't be able to answer sensibly and will just reach into the box beside them to grab a kitten to strangle instead when you ask for your ticket to be escalated to someone who actually can fix this for you. I know for certain that there are in fact very knowledgeable ppl in Garmin but you'd be lucky to get one first-up answering the support lines and you'd need to be very persistent to get put thru to one.

If you don't wish to inflict the torture on yourself of waiting endlessly on the phone to them, we can copy and then erase the device's nonvol region to see if that clears the problem. Let's know if you want to try that next.

*EDIT: Ah! I see GN has done that for you and it seems the UNL code is indeed invalid. Try recreating the UNL with [Only registered and activated users can see links. ] and replace the one supplied by GE.