I'm having the same problem. I actually e-mailed Datel about it, but they say there's nothing wrong.
I've had the LAN adapter for ages and never had a problem until recently. I've updated my Wii twice recently (Metroid and Mario Galaxy). That's when the problem started. I'm guessing a Wii system update blocked the LAN adapter out so we'd run and buy Nintendo's official adapter.
'Course, the only way to find out if I'm right is to buy a Nintendo LAN adapter, and I'd rather NOT do that.