i got this kind of problems before… but there are lots of possible solutions.
first of all, check that the LAC, CI of the external neighbors cells are defined in both OMCR and in both MSC. 50% of the case, the people in charge of OMCR or MSC will ensure you they did it correctly. Don’t trust them and ask them to check it again thoroughly.
second thing, you can ask a NSS guy to check an internal “addressing” table in the MSC, or something. Unfortunately I can’t remember what is was exactly… I just remember that the external LAC or external BSC wasn’t correctly defined in the NSS, but I’m not able to explain more.
as far as i recall, that’s it… but as i said, many things can go wrong. And most of them are linked to NSS.. sit down around a table with a NSS QoS expert and isolate the problem (problem hapenning during the handover preparation or execution ? where exactly is the failure ocuring ? which message exactly ? on which interface ?).