I tried to play old 0.3.1 world on last build (0.4dev-20120326-24953ba-win32), and when world is loading, i got an error:
ERROR[main]: ServerError: LuaError: error: ...in\..\games\minetest_gam
e\mods\default/leafdecay.lua:75: attempt to index local 'reg' (a nil value)
Wait, wait.
What is the error message you get? Judging from the little information you gave us, It might also be that a part of the map is accessed while not being loaded yet.
I think this appears for node that don't have any group. Don't know if this is a bug or not (cause all nodes must have groups for now). Maybe you use old mods?
i tried to play map that i played on december 0.4dev version. And it causes that error when i tried to move at region which hadn't loaded. Seems like it really could be a problem with gorups.