Hi Daniela,
You should generate the mesh counterclockwise, see the following example:
//
border A(t=0,0.5){x=2+t;y=0; label=1;};
border B(t=0,2.5){x=2.5;y=t;label=2;};
border C(t=0,0.5){x=2.5+t;y=2.5;label=3;};
border D(t=0,0.5){x=3;y=2.5+t;label=4;};
border E(t=0,1){x=3-1.5*t;y=3;label=5;};
border F(t=0,0.5){x=1.5;y=3-t;label=6;};
border G(t=0,0.5){x=1.5+t;y=2.5;label=7;};
border H(t=0,2.5){x=2;y=2.5-t;label=8;};
int n=10;
mesh Th=buildmesh(A(n)+ B(n)+ C(n)+ D(n)+ E(n)+ F(n)+ G(n)+ H(n));
plot(Th,wait=1
);