You shouldn't need C:CALTEMPDIR in your System EnvVar 'Path'. But you should have C:\Program Files (x86)\Calibre2\ in there.
But I doubt that's causing the problem.
About 2.5 million of people run calibre from the standard Windows install folders (Program Files and Program Files (x86), and most are from non English speaking countries, see ==>
calibre - Usage Statistics for calibre. Many would have diacritics etc in their user names.
You could try installing the Portable version of calibre - its completely self-contained. If the conversion fails using it, then that would suggest it's not a Windows related issue. Installing the Portable version will not interfere with your installed version - if you install on on a thumb drive you can use it on another computer.
BTW: I can't see your launchpad post - probably because its marked Private to protect an attachment. Not a problem.
BR