I was able to resolve the error "cannot unlock maps" that occurred after updating my Zumo XT device. I hope this information will be helpful to some.
There was diffuse information online about this. I first tried resetting the device with a full data delete, to no avail. I tried to uninstall all maps via the Garmin Express app, also to no avail. I then realized there were a few maps that were duplicated, and I entered the Zumo disc manually to delete the duplicate (the filename of each map can be found in the Basecamp app when clicking on the map). This didn't work. I then saw, on the Garmin site, that you might have to manually unlock maps in basecamp. I entered basecamp -> maps ->map options, but this fan clearly showed that all my maps were UNLOCKED. Ie, it's not a problem caused by lacking code, but probably a bad file.
I realized that the map folder didn't show all the maps that were shown in Basecamp. I used Garmin express to uninstall the included maps (north america (I live in south america)); this didn't help. After removing them I realized that two maps were still included, though I had deselected / uninstalled all maps via the Garmin express app as well as manually through deleting them from the Zumo drive -> internal memory -> Maps folder.
The two maps, at least in my case, that persisted, were: Foursquare map of north america, and National parks of north america. Both of these files had .img names that were impossible to find on the internal drive, which prompted me to believe that there is probably a hidden system folder. Some googling and, indeed, a hidden system folder (called ".system") can be accessed through developer tools on the device (search how to do this on your own responsibility; I found a guide on advrider). I enabled this, which gave me access to the hidden .system->maps folders. Beware that if you mess something up in this folder, it may cause malfunction that cannot be resolved without sending your device to Garmin physically! Either way: I was there able to localize the two maps that the garmin express app did neither detect nor uninstall. I uninstalled them both, restarted the device, and the error messages were gone.
In summary, look for maps that are either duplicate, or obsolete. In my case I had both duplicate (double south america) and obsolete files.
Hope this helps someone.
