A couple of things you could check:
- Does it work with *xls, but not with *xlsx ? This was an issue for some excel import/export modules. Make sure to download the latest version for your Mendix version as well as the compatible versions for modules that are a prerequisite.
- Do you have 'duplicate' jar files in your appfolder\userlib\ directory ? eg commons-io-2.6.jar and commons-io-2.8.0.jar. -> make a backup copy of the userlib folder (just in case) and remove all the outdated (earlier versions) for any duplicate you find.
File that clashed was a xbean.jar and xmlbean.jar file