The Microsoft Excel 2007-2013 File Type filter is unable to save the target XLSX file and throws an error: 

Failed to save target file: Object reference not set to an instance of an object.

This is also reproducible in SDL Trados Studio 2017 when using the Microsoft Excel 2007-2013 File Type.

This happens when the XLSX source file has been created in an unsupported version of Office, more specifically the Microsoft Office/Excel 2016 build number 1707 and newer. Excel files created in Microsoft Office 2016 are not officially supported by the Microsoft Excel 2007-2013 File Type.

The resolution of this issue depends on whether you are working on a WorldServer version that includes the Microsoft Excel 2007-2016 File Type (WorldServer 11.1.1. and later), or if you work in an earlier version where only the Microsoft Excel 2007-2013 File Type is available (10.4.4., 10.4.5, 11.1 and 11.1.0).

1- If you work in a Worldserver version that does not include the Microsoft Excel 2007-2016 File Type filter: 

• Download the source XLSX file to your local computer
• Open the source XLSX file with an earlier version of Microsoft Excel )2013 and earlier)
• Save the XLSX file
• Re-submit the file to a new project or add it to the existing project as a new Task. The Task in Autoerror will then need to be canceled.  To add the file as a new Task to an existing project from Explorer, follow the steps in this article: How to add a new task to an existing project in WorldServer

2- The Microsoft Excel 2007-2016 File Type filter has been introduced in WorldServer 11.1.1. If you work in a Worldserver version that does include the Microsoft Excel 2007-2016 File Type filter, use this filter to segment your source file. Remember to adjust your Project Type or Mime Type table configuration to point to the correct file type. It is recommended to apply a Filter Group /File Type Set up to your project type that contains the Microsoft Excel 2007-2016  File Type filter in order to overwrite the settings in the Mime Type table.
Once you have configured your project type and filter group/file type set up to include the latest filter, re-submit the project with the updated project type and make sure that the new filter is used to segment your XLSX file.

*Note: The Microsoft Excel 2007-2016 File Type does not have the ability to process embedded content. Therefore if the existing filter makes use of embedded content processing the best option may be to save the file in an earlier version of Microsoft Office to avoid segmentation differences. The Microsoft Excel 2007-2016 File Type is expected to include embedded content processing in WorldServer 11.4.