App uses Google Drive API to connect to Google Drive, Google Drive API has certain access limit set by Google, like number of attempts user can make within a time and number of files an user can access, when this limit exceeds app will not allowed to access Google Drive, after sometime again app will be allowed to connect again.
App shows two types of error,
1. Temporary (TIMEOUT, REQUEST_SYNC_FAILED, METADATA_QUERY_FAILED)
These are temporary errors, app will retry automatically
2.Permanent(SIGN_IN_REQUIRED)
This error needs users intervention to resolve it, like SIGN_IN_REQUIRED will be shown if Google requests user to sign-in again.
As of release 3.3.3 and above, these error and all sync attempts can be viewed in Cloud settings (tap Last sync status to view last 20 attempts of sync)