Try to change this line

<cellStyles count="1"> <cellStyle name="Normal" xfId="0" builtinId="0"/> </cellStyles>

to

<cellStyles count="1"> <cellStyle name="Normal" xfId="1" builtinId="0"/> </cellStyles>

but I am not an expert on the xlsx xmls... Try to take time and study with excel app (and unzipping xlsx) what the underlying xmls actually represent.