Found via search, hit the same issue, in my case solution was go into Control Panel, Language Settings (region and language, etc.) click the ADMINISTRATIVE tab, click "Change system locale..." there it says:
"select which language (System locale) to use when displaying text in programs to do not support unicode."
select the local language here, click OK, reboot if needed, and xnview now able to open all the files it did not before. This also explains why it works on some PCs and not on others