New firmware looks for digital signature in maps, unlocked maps don't have it.
You can find on forum some"patched" maps for new protected devices. Some recent topo maps for example.
Devices with protected firmware: Nuvi 55/56/65/66/25x9/26x9, zūmo 590, GPSMAP 64, Edge 1000
Edit: nuvi2598 is in list too.
Edit2: in newer nuvis devices and firmware , patched maps don't work too. But try with gimgunlock+mapsigpatch from: [Only registered and activated users can see links. ]