@Lars - couple of clarifying questions:
1) Is this client roaming into range of this AP or more or less static (in other words, did it start out connected to the distant AP?)
2) What client? (OS, wireless card, etc)
3) This is only a problem since the upgrade? If yes - verify the AP took the upgrade (log into the AP). We've seen some weirdness with incomplete upgrades
3a) Did you follow the full upgrade path in the upgrade?
4) What model AP?
Thx