multicontact and 500 server error

Hi guys i've a strange problem occuring in my enviroment....my customer has several phones distributed on different realms on access side , and an Siemens OpenScape for core side .

The problem that randomly when a group of endpoint that are part of the same multicontact group (a kind of forking) register on OpenScape then SBC will not register them on his registration-cache .

So when Openscape send NOTIFY for BLF , SBC answers with a 500 global error...then Openscape try to resend NOTIFY every 4 second receiving always that kinf of error responde..this is a problem becouse it is generating a lot of traffic , do you have any idea of what kinf of problem it can be ?