Comment # 15 on bug 958682 from Wolfgang Bauer

Well, the main problem seems to be this:
(In reply to Amnon Yekutieli from comment #8)
> LANG=C.UTF-8
> LC_CTYPE="C.UTF-8"
> LC_NUMERIC="C.UTF-8"
> LC_TIME="C.UTF-8"
> LC_COLLATE="C.UTF-8"
> LC_MONETARY="C.UTF-8"
> LC_MESSAGES="C.UTF-8"
> LC_PAPER="C.UTF-8"
> LC_NAME="C.UTF-8"
> LC_ADDRESS="C.UTF-8"
> LC_TELEPHONE="C.UTF-8"
> LC_MEASUREMENT="C.UTF-8"
> LC_IDENTIFICATION="C.UTF-8"
> LC_ALL=

A similar problem was also discussed here:
https://forums.opensuse.org/showthread.php/512803-Can-t-open-file-if-the-folder-is-using-a-odd-letter
Changing the locale to something proper seems to have fixed it.

So I think the question is, why is the locale set to 'C', and where does this
come from?

IMHO, not a bug in KDE though.