I don't think this has anything to do with ATK. The error messages come from the system ("Stale NFS file handle"), perhaps you should run a disk check.
You don't state what the actual problem is with the band structure calculation (error message?) but if you repeat the system, the class for the BravaisLattice is changed from Hexagonal to UnitCell, and then the symmetry points are not recognized any more. You can however change it back to Hexagonal using Bulk Tools>Lattice Parameters in the Builder, after the repeat has been applied.