Code as below. I think it is due to a non positive definite covariance input. When this is fixed, all is fine.

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require(OpenMx)

varnames<-c("BIC1","SSC1","BIC2","SSC2");

dataMZ<-dataDZ<-matrix(diag(1,4),nrow=4,ncol=4,dimnames=list(varnames,varnames));

dataDZ[1,2]<-dataDZ[2,1]<-1.2;

p<-2;

pstar<-p*(p+1)/2;

diagind<-(diag(p)==1);

nMZ<-84;

nDZ<-33;

varnames<-c("BIC1","SSC1","BIC2","SSC2");

LA<-mxMatrix(type="Lower",nrow=p,ncol=p,free=T,values=0,

label=paste("a",1:pstar,sep=""),name="LA");

LC<-mxMatrix(type="Lower",nrow=p,ncol=p,free=T,values=0,

label=paste("c",1:pstar,sep=""),name="LC");

LE<-mxMatrix(type="Lower",nrow=p,ncol=p,free=T,values=c(1,.5,sqrt(3)/2),

label=paste("e",1:pstar,sep=""),name="LE");

LA@lbound[diagind]<-0;

LC@lbound[diagind]<-0;

LE@lbound[diagind]<-0;

A<-mxAlgebra(expression=LA%*%t(LA),name="A");
C<-mxAlgebra(expression=LC%*%t(LC),name="C");

E<-mxAlgebra(expression=LE%*%t(LE),name="E");

ACESigmaMZ<-mxAlgebra(expression=rbind(cbind(A+C+E,A+C),cbind(A+C,A+C+E)),

name="ACESigmaMZ");

ACESigmaDZ<-mxAlgebra(expression=rbind(cbind(A+C+E,.5%x%A+C),cbind(.5%x%A+C,A+C+E)),

name="ACESigmaDZ");

ACEModelMZ<-mxModel("ACEModelMZ",

mxMLObjective(covariance="ACEModelTwin.ACESigmaMZ",dimnames=varnames),

mxData(observed=dataMZ,type="cov",numObs=nMZ));

ACEModelDZ<-mxModel("ACEModelDZ",

mxMLObjective(covariance="ACEModelTwin.ACESigmaDZ",dimnames=varnames),

mxData(observed=dataDZ,type="cov",numObs=nDZ));

twin<-mxAlgebra(expression=ACEModelMZ.objective+ACEModelDZ.objective,name="twin");

ACEModelTwin<-mxModel("ACEModelTwin",LA,LC,LE,A,C,E,

ACESigmaMZ,ACESigmaDZ,ACEModelMZ,ACEModelDZ,twin,

mxAlgebraObjective("twin"));

mxRun(ACEModelTwin)

This is a bug in the backend. Your intuition is correct, it is caused by a non-positive-definite observed covariance matrix. However we definitely shouldn't be throwing a segmentation fault in these cases. We're working on a fix.