And here is the current response from Telerik on the matter:
-----------------

I have inspected your document and I found that it contains style definition with number format id value 43 which is built-in format but in the XLSX file format specification is not described and that is why we have not implemented it. This is causing the exception you have observed.

Here is the original message, that was handled with a support ticket. The description above doesn't say much, so I thought I'd add this as information to other developers:
---------------

I got a xlsx-file that was saved LibreOffice Calc 4.4.5.2 and when I tried to open it with SpreadProcessing I got this exception:

Exception Details: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.

Stack Trace:

[KeyNotFoundException: The given key was not present in the dictionary.]
   System.Collections.Generic.Dictionary`2.get_Item(TKey key) +11000705
   Telerik.Windows.Documents.Core.Data.QueueDictionary`2.get_Item(TKey key) +46
   Telerik.Windows.Documents.Core.Data.ResourceIndexedTable`1.get_Item(Int32 index) +47
   Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Contexts.XlsxWorkbookImportContext.CreateStyleFromStyleInfo(StyleInfo styleInfo, CellStyle builtInStyle) +837
   Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Contexts.XlsxWorkbookImportContext.ImportStyles() +325
   Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.Model.Elements.StyleSheet.StyleSheetElement.OnAfterRead(IXlsxWorkbookImportContext context) +160
   Telerik.Windows.Documents.FormatProviders.OpenXml.Model.Elements.OpenXmlElementBase`3.OnAfterRead(IOpenXmlImportContext context) +90
   Telerik.Windows.Documents.FormatProviders.OpenXml.Model.Elements.OpenXmlElementBase.Read(IOpenXmlReader reader, IOpenXmlImportContext context) +241
   Telerik.Windows.Documents.FormatProviders.OpenXml.Model.Parts.OpenXmlPartBase.Import(IOpenXmlReader reader, IOpenXmlImportContext context) +60
   Telerik.Windows.Documents.FormatProviders.OpenXml.OpenXmlImporter`1.ImportXlsxPartFromArchive(ZipArchiveEntry zipEntry, OpenXmlPartBase part, IOpenXmlImportContext context) +242
   Telerik.Windows.Documents.FormatProviders.OpenXml.OpenXmlImporter`1.Import(Stream input, IOpenXmlImportContext context) +1064
   Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.XlsxFormatProvider.ImportOverride(Stream input) +119
   Telerik.Windows.Documents.Spreadsheet.FormatProviders.WorkbookFormatProviderBase.Import(Stream input) +120


I re-saved the file in Excel 2013 and it worked.

This is the code I use to open the file:
using (var fs = new FileStream(Path.Combine(physicalUploadFolderPath, excelFilename), FileMode.Open))
{
    XlsxFormatProvider formatProvider = new XlsxFormatProvider();
    return formatProvider.Import(fs);
}


The exception occurs at Import(fs).


Attached files:
export.xlsx is created in Libre Office Calc.
export2.xlsx is created in MS Excel 2013.