I have similar issues. Sometimes when I attempt to log in my phone recognizes the face ID (iPhone X), which is a substitute for entering the VIPS password. Then it still asks for the VIPS password again and crashes.
My more frequent problem is it will login correctly, but then crash when I try to refresh my calendar or a pairing. It only does it once though. If I repeat the process again everything works correctly. It's definitely a problem with the app, not the FedEx servers. This one is so common I've just come to accept that any time I use the app I have to let it crash once before I can use it.