Это ожидаемо, неверная URL.
https://tools.ietf.org/html/rfc3986
In local or regional contexts and with improving technology, users
might benefit from being able to use a wider range of characters;
such use is not defined by this specification. Percent-encoded
octets (Section 2.1) may be used within a URI to represent characters
outside the range of the US-ASCII coded character set...
Пример кодирования
file:///C:/привет/по%20русски.txt
file:///C:/%D0%BF%D1%80%D0%B8%D0%B2%D0%B5%D1%82/%D0%BF%D0%BE%20%D1%80%D1%83%D1%81%D1%81%D0%BA%D0%B8.txt
This posting is provided "AS IS" with no warranties, and confers no rights.
WebBrowser не переходит в папку С# WPF
