The "special://" URI prefix maps to the root of where Kodi writes data, e.g. /storage/.kodi .. but that's a Kodi internal thing so firstly OpenVPN doesn't understand it, and second askpass doesn't accept URIs as input only paths.
If an absolute /path/to/the/file isn't working it might be relative, e.g. if JimH.ovpn and user.txt are in the same folder, only add the filename alongside 'askpass' and don't add the path.
If that's not it you probably need to look into (or share) the OpenVPN log. I've never used that add-on but I'd guess it generates a log somewhere and that might have more info.