Randomly, my email app does this (I won't use the official Gmail app as I hate the UI plus the default email app in Android supports multiple accounts). But my password and username is correct. I set the sync interval to four hours, and only access this account from my one phone. Every time this error comes up, I have to clear data from the app which means manually adding each account back. It is starting to get annoying. Is this related in any way to the mass of Google errors affecting Play Store and other Google apps lately?