For those wondering, I've had the exact same problem on both Mindcrack 8.0.1 and 8.1.1. So basically, I go into the twilight on an SMP server and make the magic map. I go to use it and it will create a blank map for me to fill. Sounds cool, right? Until you realise it will stay like this:
(image removed to save space)
I've even gone for a wander and it's done nothing. Any suggestions?
Magic Maps are a bit buggy, sometimes not rendering at all when you're in range of the map's viewing radius and often not rendering landmarks icons properly, and something tells me they probably work quite worse on SMP servers where everyone would want to make a Magic Map and find a bunch of Hollow Hills to get tons of ores.
Make sure that you are indeed within the range of the map's mappable area. If you are, then relog, drop the map a few times, put it in a chest and come back after a minute or two of doing nothing, and so forth to make sure it's not a client-side bug. If you aren't or if that doesn't work, try making/centering (right-clicking with blank map in hand to center) a new map at a far-away location, as your Magic Map's centering point may be "gravitating" to an already-used centering point and bugging out because of it. Not sure why, but this is Twilight Forest we're talking about; a great and well-balanced mod (save the Magic Map, which is pretty OP) but way buggy nonetheless.
You might also want to check the numbers of the maps you've made and see if anyone else on the server has any with the same number, as that could be causing problems. If there are people with the same Magic Map, try copying the Magic Map by placing the completed/semi-completed one in a crafting grid with a blank one. That's something I've not experimented with, and I'm not sure if it's possible, but give it a shot nonetheless.
Unfortunately, Twilight Forest isn't all that well-documented even on its own wiki, so it's hard to tell what's a bug, what's intentional, et cetera, but this seems to be some sort of bug, and hopefully one that has a fix or workaround.