MA111 only works with one AP

I've got a MA111 running on a Dell Inspiron 5000 with a custom 
2.6.12-cko2 kernel and wlan-ng 0.2.1-pre26. I have two 802.11g APs - one 
is a 3Com, and the other is home-made, running Pebble Linux on an old 
P166, with an Atheros PCI card in Master mode. From the laptop, I can do 
iwlist and lnxreq_wlansniff, and I can pick up both APs, as well as my 
neighbour's. However, I can only associate with the home-made Atheros 
AP. When I try to get on to the 3Com, I get the ASSOCFAIL message, 
although the 3Com's log (about as useful as last week's unsuccessful 
lotto numbers) reports a connection from the MA111's MAC.

Clearly, I can't change anything significant on the 3Com - I've already 
disabled WEP and MAC filtering - but I'm wondering are there any 
possibilities among the many commands and/or MIBs that would be worth 
trying?

Any suggestions/ideas appreciated. TIA