Show a message and block editing (or logout) when the credentials become invalid
After change your password on the Web app, the mobile app stops syncing and never shows any message saying that the credentials become invalid.
My wife was entering several task in the mobile app and she noticed that the tasks were erased milliseconds after she entered each task but she thought that it was a refresh issue and her tasks were saved.
Today I saw that she had an old list and then I realized that the mobile app wasn't syncing because I changed the password a few days before.
I think that will be helpful that the mobile app raises a message and block the editing (or logout) when the credentials become invalid.