@ningosi, I thought your problem was in loading of the Excel template, not in writing it out or trying to open a rendered report (see your stack trace).
Based on your stack trace, the right line of code is being called to attempt to load this template as an xlsx file. Where exactly are you experiencing this issue? Are you setting up reports in code that are then later failing to render properly? Or is this happening when you upload xlsx templates from the admin UI?
If it is happening after you set things up in code, my guess is that your template is getting corrupted along the way. Often this will happen if you are doing resource filtering during the maven build process and you are not limiting this to the appropriate types of files. One way to confirm whether this is indeed the case, is to build your module with maven as normal, and then go into target/classes and find the xslx file that you are trying to set up in the compiled class folder. If you try opening this here and it fail to open due to some corruption, then you know the issue is in the build process and not with the reporting module itself.
Give it a go and let me know how you end up.
Mike