As Apple ups its game with two-factor authentication (you are using it, right?), you may find some of your apps suddenly stop recognizing your iCloud password. I've seen this in my life with Sanebox, Fantastical, BusyCal, and Dispatch just to name a few. In these instances, Apple stops recognizing your password and instead requires you to make an app-specific password just for that third party application. If you suddenly find yourself locked out of third party email, calendar, and other iCloud account related applications, you'll need to learn to set up your own app-specific passwords. It is not that difficult.
It all starts right here at the My Apple ID page.
You are going to want to bookmark the above link. You'll need it often as you are required to create app-specific passwords. The My Apple ID page gives you a few options. Click on "Mange your Apple ID".
Next, you'll need to insert your account credentials.
You'll also need to provide two factor verification to get in.
Once your in, click on the security pane and the app-specific passwords are available at the bottom of the right pane.
Click on "Generate an App-Specific Password" and you'll walk through the process. First you'll name the App-Specific password. Name it after the app you intend to use it in. Then iCloud will spit out a new password. Use that in your designated third-party application and you're in.
You can even access (or revoke) prior app-specific passwords. While a bit inconvenient, this is absolutely do-able. Moreover, having app-specific passwords is much more secure. Given the amount of data and information in my iCloud account, the increased security is worth the time this takes to set up.