A tale of two D-link DI-524's

I got two routers in different locations. One is in my place, and one is
in my mother's house (of which I happen to be the defacto sysadmin).
They are both the same C-stepping of DI-524 and I've installed the same
firmware between them.

However, for some reason, the one in my mother's house has some trouble
with the WiFi connection. The only way I can get it to work is to turn
off all WPA or WEP encryption and access it unsecured. I have no idea
why two such identical routers behave so differently.

Yousuf Khan