For various reasons it's still necessary to be able to work in a non-UTF8 locale (in my case de_DE@euro with the ISO-8859-15 character set). Dolphin 17.04.2 under OpenSuse 42.3 worked correctly in this locale, but Dolphin 18.04.1 under OpenSuse Tumbleweed doesn't. Diacritical characters in filenames are shown as a question mark in a black square and selecting such files or directories isn't possible, because dolphin says, that they don't exist.
I've just seen, that this issue affects not only dolphin, but other other KDE tools, too. It seems, that it's not possible any more to open files with diacritical characters in any KDE tool. So it's most likely a KIO framework bug. I've changed the Product entry accordingly.