After inziping XLSX, in the _rels folder, you'll find the .rels XML file. In it, you'll find somewhere with the Target tag, and the contents is "/docProps/core.xml". This is the problem. LibreOffice can only accept the Target tag with a value of "docProps/core.xml". Note the first slash character. If you remove the starting slash character, then re-zip the whole thing, rename as XLSX, LibreOffice opens it fine. Soulution: Fix "docProps/app.xml" and "docProps/custom.xml" and "/docProps/app.xml" and "/docProps/custom.xml" must do the same! Funnily, LibreOffice accepts "/xl/workbook.xml" for the workbook part. So I dont know why dont accept slash before "docProps/app.xml" and "docProps/custom.xml".
This is far from being a critical or even a blocker bug. While it is annoying it seems that your file is not created by MS Excel.
This file IS NOT generated by Excel but it is generated by OpenDocument SDK from Microsoft (.NET library for generating XLSX files)
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Martin: is this still a problem for you? If yes, attach an example file as this has not been confirmed by QA yet.
Dear Bug Submitter, Please read this message in its entirety before proceeding. Your bug report is being closed as INVALID due to inactivity and a lack of information which is needed in order to accurately reproduce and confirm the problem. We encourage you to retest your bug against the latest release. If the issue is still present in the latest stable release, we need the following information (please ignore any that you've already provided): a) Provide details of your system including your operating system and the latest version of LibreOffice that you have confirmed the bug to be present b) Provide easy to reproduce steps – the simpler the better c) Provide any test case(s) which will help us confirm the problem d) Provide screenshots of the problem if you think it might help e) Read all comments and provide any requested information Once all of this is done, please set the bug back to UNCONFIRMED and we will attempt to reproduce the issue. Please do not: a) respond via email b) update the version field in the bug or any of the other details on the top section of FDO Message generated on: 2015-02-11