Login Error 53021
When a user attempts to access the site they get a 53021 error on login.
When a user has provided too many bad passwords the account and/or IP address will be locked as per the Account Lockout Policy.
The error will appear to like this: 
To clear a locked user or IP address perform the following:
1. Log into the site as a administrator.
2. Go to Settings > Authentication > Account Lockout Policy 
3. Click the Trash Can icon and this will remove the lockout.