Problems loading in biaxial tension

Hello all,
Nice to meet you, I'm Guy, a postdoc from the University of Manchester, UK. I have had some problems in attempting to simulate biaxial loading using VPSC8:
Please see the following boundary conditions I am attempting to impose:

10 0 0.01 298. 298. nsteps  ictrl  eqincr  temp_ini   temp_fin
* boundary conditions           iudot    |    flag for vel.grad.
1       1       1
1       1       1
1       1       0
*                               udot     |    vel.grad (first guess)
1       0.0     0.0
0.0     1       0.0
0.0     0.0     -2
*                               iscau    |    flag for Cauchy
0       0       0
        0       0
                1
*                               scauchy  |    Cauchy stress
0.0     0.0     0.0
        0.0     0.0
                0.0

I'm trying to apply a load in biaxial tension in the xx and yy directions but leave the zz direction free to move with zero stress.
We've been able to run this in VPSC7 previously.
When attempting to run this with VPSC8 I get the following error:

 ***************************************************
 SINGLE ELEMENT CALCULATION
 SINGLE PHASE CALCULATION
 ***************************************************
 --> TOTAL NUMBER OF GRAINS IS  2000

  CHECK DIAGONAL BOUNDARY CONDITIONS ILBAR:
  ENFORCING TWO DIAGONAL COMPONENTS FIXES THE
  REMAINING COMPONENT BECAUSE OF INCOMPRESSIBILITY

This is understandable but not incompressible? The zz direction is allowed to contract to allow for the expansion in the xx and yy directions.
Please find attatched all of my files in my attempt to run this:
VPSC_biaxial_issue.zip

Thank you,
Guy

Commencez par décompresser l’archive jointe VPSC_biaxial_issue.zip et exécuter VPSC8 avec l’entrée de conditions aux limites fournie. Suivez la vérification des conditions aux limites diagonales signalée et comparez les contraintes xx, yy et zz configurées avec le chargement prévu ; le travail est terminé lorsqu’il est déterminé si l’entrée ou le comportement de VPSC8 empêche la contraction libre selon zz et que la correction requise est documentée.