Object reference not set to an instance of an object. XLS. Trados 2017

 This is the error script I get. I understand many people have got problems with Saving file as target, I keep on getting the same problem with XLS; it had never happened before these last few days and I wish I knew why. Help anybody? Kind regards.

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<?xml version="1.0"?>
<SDLErrorDetails time="07/09/2017 10:13:54">
<ErrorMessage>Failed to save target content: Object reference not set to an instance of an object.</ErrorMessage>
<Exception>
<Type>System.NullReferenceException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>
<HelpLink/>
<Source>Sdl.FileTypeSupport.Filters.Excel</Source>
<HResult>-2147467261</HResult>
<StackTrace>
<![CDATA[
at Sdl.FileTypeSupport.Filters.Excel.FilterControllerFramework.OfficeController.UpdateDocumentFile(XsltArgumentList argsList, XslCompiledTransform xslTransform, XPathNavigator child)
at Sdl.FileTypeSupport.Filters.Excel.FilterControllerFramework.OfficeController.IterateXsltFiles(XsltArgumentList argsList, FileMap rule, XPathNodeIterator iter, Boolean isUpdate, String targetFilePath)
at Sdl.FileTypeSupport.Filters.Excel.FilterControllerFramework.OfficeController.IterateContentType(String xslFileNamesPath, Boolean isUpdate, String filePath, String filterStr, String targetFilePath, Boolean isCalledByRelsTableIteration)
at Sdl.FileTypeSupport.Filters.Excel.FilterControllerFramework.OfficeController.serviceBackward()
at Sdl.FileTypeSupport.Filters.Excel.PlugInFilter.FileParser.DocumentParser.Update()
at Sdl.FileTypeSupport.Filters.Excel.PlugInFilter.FileParser.DocumentExtractImpl.Update(DocumentCollection srcCollection, String filename, String tgtFilePath)
at Sdl.FileTypeSupport.Filters.Excel.PlugInFilter.FileWriterAndPreviewFileWriter.WriterImpl.EndOfInput()
at Sdl.FileTypeSupport.Filters.Excel.PlugInFilter.FileWriterAndPreviewFileWriter.ExcelWriter.EndOfInput()
at Sdl.FileTypeSupport.Framework.Integration.NativeWriterMessagesProxy.EndOfInput()
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