Apple requires all third-party apps that sync with iCloud (like BusyCal and BusyContacts) to use app-specific passwords - your standard iCloud password will not work. An app-specific password is essentially a 16 to 19 characters long, custom password generated by Apple that is different than your standard iCloud password. Only Apple's apps (Mail, Calendar and Contacts) can use your standard iCloud password. All other apps must use an app-specific password to connect with your iCloud account.

Also please note, if you have created an "alias" for your iCloud email address, that will not work even when used with an app-specific password. Please make sure you're using your real iCloud account email address.

Apple has explained how to generate an app-specific password here

If you are syncing BusyCal or BusyContacts with iCloud, please follow the instructions below to set up an app-specific password. If you are already using an app-specific password in BusyCal or BusyContacts, you can skip the process below.

1. Enable two-factor authentication

Note: If you have already enabled two-factor authentication, you can skip to step 2.

In order to create an app-specific password for BusyCal and BusyContacts, you must first enable two-factor authentication for your iCloud account. Two-factor authentication requires at least one iPhone, iPad, or iPod touch running iOS 9 or later, or a Mac running OS X El Capitan or later.

To enable two-factor authentication on a Mac:

• Go to Apple () menu > System Preferences > iCloud > Account Details.
• Click the Security tab.
• Click the Turn on Two-Factor Authentication button.

For more info, see Apple's help page: Two-factor authentication for Apple ID.

2. Create an app-specific password

To create an app-specific password for BusyCal or BusyContacts:

In your web browser:

• Sign in to your Apple ID account page with your web browser.
• In the Security section, under App-Specific Passwords, click Generate Password...
• Enter a label for the password (e.g. BusyCal) and click Create.
• An app-specific password will be generated and displayed onscreen. Select the password and copy it to your clipboard. Also, make sure you write down the password or save it in a password manager. You will need it for other Macs or iOS devices running BusyCal or BusyContacts, and you may need it again in the future.

3. Enter the app-specific password in BusyCal or BusyContacts

To enter the app-specific password in BusyCal or BusyContacts:

• Select BusyCal > Preferences > Accounts, select your iCloud account, then enter or paste the app-specific password into the password field. This should be a 16-19 character long password. Or you can enter the new app-specific password in the BusyCal Login dialog when prompted.

The process for using an app-specific password in BusyContacts is the same as BusyCal.

Note: You do not need to create separate app-specific passwords for each app and device. The same app-specific password can be used for multiple apps (BusyCal and BusyContacts) and multiple devices (Mac and iOS) as long as they are all connecting to the same iCloud account.

If you have forgotten your iCloud app-specific password, you can always generate a new one by following steps mentioned here

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