As I can see Libre Office is normally open files in rare pre-standardized Office Open XML 2007 format that OpenOffice can’t open correctly. Which lib used to open such a files?
It is not entirely clear from the question exactly which file format is being referred to however it is possible that “rare pre-standardized Office Open XML 2007” is a reference to the Flat OPC 2007 XML format. If this is the case it is an unsupported format (refer this thread).
Alternatively, if this is a reference to early variants of OOXML (ECMA-376 / ISO 29500) then it is likely the MS Word 2007 XML (Microsoft Word 2007-2013 XML) filter is used. List of filters here.