[Derision]

From Cavedude's backtrace, it looks like maybe it has something to do with pets, in this code around line 505 in groups.cpp:

#ifdef GROUP_BUFF_PETS
                                if(members[z]->HasPet())
                                        members[z]->GetPet()->BardPulse(spell_id, caster);
#endif

Maybe getting a NULL pointer from GetPet() ? (this=0x0 in Mob::BardPulse in Cavedude's BT)

Could try checking if GetPet() returns NULL which may stop the crashes (although not fix the root cause):

#ifdef GROUP_BUFF_PETS
                                if(members[z]->HasPet()) {
                                        Mob* tmppet;
                                        tmppet = members[z]->GetPet();
                                        if(tmppet != NULL) tmppet->BardPulse(spell_id, caster);
                                }
#endif

I've not tried to reproduce this or anything, just throwing this out there as a possible avenue of investigation.