The '\' is an unsafe character according to RFC1738, and it should not be used in an URL. Unfortunately it's not handled correctly by WAF at the moment. Since '\f' is an ASCII control character, WAF treats it as such. This is a legacy problem, it can be reproduced both on v18 and v17.5.
We're tracking it internally as NC-53175.
You can probably use the '/' character as a substitute in the URL if you are trying to configure a path to a directory called fow inside ~NS.
