Do you have a test mobile like the Sagem OT260? I think it has the capability of forced handover.If you can find it try the following:
1)Prepare SS7 signalling(BSSAP and MAP) at both the BSCs and the MSC.
2)Go to one of the cells with the 100% HO failure.
3)Try to make a call and then apply the forced handover to one of the cells on the other BSC.
4) If the handover is unsuccessful then go back and see your traces, then you may see the error. If you are unfamiliar with the ss7 messages MAP and BSSAP messages, you can make a handover in a normal cell then record the messages in line. Compare the messages of the faulty cells with the normal and see the difference. Some GSM vendors they decode the messages and it is very easy to understand but I don’t know about Nokia because I don’t have experience on it.