Typically, the first thing to do is to wipe the app cache, then uninstall and reinstall the app.
This process is a bit longer than just dumping and reinstalling, but it will completely remove any trace of the app and it's data, which may be causing your issue. Best to start cleanwith the fresh app install.
1. Open SETTINGS, then find APPLICATIONS (or APPS).
2. Find and tap on your AMEX app. Tap FORCE STOP. If you get a message that it will make the app stop working, acknowledge it.
3. Tap STORAGE, then CLEAR DATA.
4. Tap UNINSTALL to remove the app.
5. Restart your phone, then reinstall the app.