If new users cannot log in, the biggest reason may be that the user directory hasn't synced after adding the user. For instance, a user added at noon will not be able to log in if the last directory sync was at 8:30 a.m. Without being synchronized, the user is not in the table and cannot log in.
To synchronize the user directory:
- Login to Backpack.
- Click the Settings card.
- On the User Directory Settings card at the left, click Sync User Directory.