TYPO3 Core

if you look at the stacktrace you see that de fileIndentifier is somewhere trimmed. The first char is removed

The problem lies in PHPs function basename() which is used in the PathUtility:

public static function basename($path)
    {
        $currentLocale = setlocale(LC_CTYPE, 0);
        setlocale(LC_CTYPE, $GLOBALS['TYPO3_CONF_VARS']['SYS']['systemLocale']);
        $basename = basename($path);
        setlocale(LC_CTYPE, $currentLocale);
        return $basename;
    }

If your locale is not set to UTF-8, basename() will return an empty string on filenames that start with an umlaut.
To fix this behaviour, set your systemLocale in TYPO3 to some string with UTF-8.

Example:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['systemLocale'] = 'en_US.UTF-8';

I reported it for TYPO3 9.5.5 - now I tested again on a TYPO3 8.7.27 and TYPO3 9.5.9

Both have (and had) the same settings:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['UTF8filesystem] = true;
$GLOBALS['TYPO3_CONF_VARS']['SYS']['systemLocale'] = 'de_DE.UTF-8';

I can no longer reproduce the issue, maybe it was solved in between? Frans, could you please confirm the issue is no longer there for you too?