You probably want to try the URL https://online.americanexpress.com/myca/ofxdl/desktop/desktopDownload.do?request_type=nl_ofxdownload instead of https://www99.americanexpress.com/myca/ofxdl/us/download?request_type=nl_desktopdownload .
For me the login seems to work with this.

OFX REQUEST https ...