During test it was found that a router uses the current password with a slash "/", that is the URI delimiter.
Is it possible to pass such password with this slash to http://192.168.1.4/set/ssid/pass_slash_word ?
The proper solution is to url encode the parameters and decode them on the Arduino.
I don't remember the server code but it shouldn't be too difficult to make it get the string after ssid/ (it will work with slashes and still fail in other cases where the browser encodes the characters).