I like that WordPress creates a slug automatically from post titles by setting it to lower-case and removing all special characters. However, there seems to be (at least) one replacement mistake.
'ß' is replaced by 's'. Although the letter (special German one) is called 'sz' for historical reasons, it is canonically replaced by 'ss' if 'ß' is not available (font, upper case, ...).
Please change the responsible rules accordingly.