I am working with SDL Trados Studio 2015 SR3 - 12.3.5281.10 and when trying to save an Excel-file as target I receive the following error message (I paste the entire message):

<SDLErrorDetails time="11/24/2017 12:42:18 PM">
  <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()
   at Sdl.FileTypeSupport.Framework.Integration.FileGenerator.CallEndOfInput()
   at Sdl.FileTypeSupport.Framework.Integration.FileGenerator.FileComplete()
   at Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.FileComplete()
   at Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.FileComplete()
   at Sdl.FileTypeSupport.Framework.BilingualApi.AbstractBilingualContentProcessor.FileComplete()
   at Sdl.FileTypeSupport.Framework.Integration.AbstractBilingualProcessorContainer.FileComplete()
   at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReaderImpl.vv_Visit(TmfcReaderImpl* , FrameworkFileContainerField* field)
   at FrameworkFileContainerField.vv_AcceptFrameworkVisitor(FrameworkFileContainerField* , FrameworkFieldVisitor* visitor)
   at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReaderImpl.vv_VisitStartEndField(TmfcReaderImpl* , StartEndField* startEnd)
   at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReaderImpl.b_VisitNext(TmfcReaderImpl* )
   at Sdl.FileTypeSupport.Bilingual.Tmfc.TmfcReader.ParseNext()
   at Sdl.FileTypeSupport.Framework.Integration.FileExtractor.ParseNext()
   at Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.ParseNext()
   at Sdl.FileTypeSupport.Framework.Integration.MultiFileConverter.Parse()
   at Sdl.TranslationStudio.Editor.TranslationEditor.TranslatableDocument.SaveMonolingualAsJobRequest.Execute(IJobExecutionContext context)
   at Sdl.Desktop.Platform.Implementation.Services.Job.<_worker_DoWork>b__3()
   at Sdl.Desktop.Platform.Implementation.Services.Log.Resources(Object message, Action action)
   at Sdl.Desktop.Platform.Implementation.Services.Job._worker_DoWork(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)]]></StackTrace>

Do you have any idea what could be the Problem?

Thank you in advance for your help.

Martin