[Derision]

This makes no sense to me. If we consider frame 3

#3  0x0000000000719042 in EntityList::CheckClientAggro (this=0xbdcfa0,
    around=0x7feab85bb4b0) at aggro.cpp:45

This is the code from aggro.cpp around that line:

  39         for(iterator.Reset(); iterator.MoreElements(); iterator.Advance()) {
  40                 _ZP(EntityList_CheckClientAggro_Loop);
  41                 Mob* mob = iterator.GetData();
  42                 if(mob->IsClient())     //also ensures that mob != around
  43                         continue;
  44
  45                 if(mob->CheckWillAggro(around)) {

So in your backtrace, mob->CheckWillAggro is called at line 45, but you are saying that both mobs involved are players, but line 42 checks if mob is a client and will skip line 45 and move on to the next mob if that is the case.

Strange.

I note that both the BTs you posted are for kerraridge. Do all the crashes happen there ?

Have you talked to the players involved in the crashes to see what they were doing at the time the zone crashed ?