I took your data and written them in ascii format using the vtkXMLUnstructuredGridWriter.
<?xml version="1.0"?>
<VTKFile type="UnstructuredGrid" version="0.1" byte_order="LittleEndian" header_type="UInt32" compressor="vtkZLibDataCompressor">
<UnstructuredGrid>
<Piece NumberOfPoints="12" NumberOfCells="2">
<PointData>
</PointData>
<CellData>
</CellData>
<Points>
<DataArray type="Float64" Name="points" NumberOfComponents="3" format="ascii" RangeMin="19.088177671081006" RangeMax="42.22372218524202">
10.402215058538264 7.019119310491388 38.34784825517838 2.071068325188502 2.9289322463883574 42.07106781465104
8.533894273043977 2.7643394369899603 29.493119254777678 0.20274753969421272 -1.3258476271130704 33.216338814250335
6.665573487549688 -1.4904404365114674 20.63839025437698 -1.6655732458000754 -5.580627500614498 24.36160981384964
5.196317292553462 15.091740818322124 35.56730686159636 -3.134829440796301 11.001553754219094 39.29052642106902
3.327996507059174 10.836960944820698 26.71257786119566 -5.00315022629059 6.746773880717667 30.43579742066832
1.4596757215648852 6.58218107131927 17.85784886079496 -6.8714710117848785 2.491994007216239 21.58106842026762
<InformationKey name="L2_NORM_RANGE" location="vtkDataArray" length="2">
<Value index="0">
19.088177671
</Value>
<Value index="1">
42.223722185
</Value>
</InformationKey>
</DataArray>
</Points>
<Cells>
<DataArray type="Int64" Name="connectivity" format="ascii" RangeMin="0" RangeMax="9">
6 7 9 8 0 1
3 2
</DataArray>
<DataArray type="Int64" Name="offsets" format="ascii" RangeMin="8" RangeMax="8">
8 8
</DataArray>
<DataArray type="UInt8" Name="types" format="ascii" RangeMin="12" RangeMax="12">
12 12
</DataArray>
</Cells>
</Piece>
</UnstructuredGrid>
</VTKFile>
Since your cell types are hexahedrons, i would expect 8+8=16 connectivity entries, and i only see 8. So that’s one mistake.
Since your cells are 2 hexahedrons, i would also expect 0, 8, 16 as offsets, and i see 8,8 and that’s another mistake.
I hope that this answers your question and helps you fix your dataset.