itune logs in fine, but icloud keeps giving authentication error

Hello Possum53,

Welcome to Apple Support Communities. 

We understand that you are getting an error when you try to log in to iCloud for Windows. We are happy to help.

We’d like to have you start with restarting your computer. Once your computer has fully restarted, are you able to login to iCloud for Windows? Do you have any security software, such as antivirus software or a VPN? If so, we recommend that you disable those and then restart your computer and see if you can sign in to the app now? Please check the Microsoft App Store for any available app updates, as well as updates fro your computer.

The Apple system Status is not showing any outages: Apple - Support - System Status

Here’s more about setting up iCloud for Windows: Set up iCloud for Windows on your PC - Apple Support

"Set up iCloud for Windows

1. If you don’t already have it, download iCloud for Windows from the Microsoft Store. See the Apple Support article Download iCloud for Windows.
2. On your Windows computer, open iCloud for Windows.
3. Sign in with your Apple ID.
4. Make sure to sign in with the same Apple ID that you use on your iPhone, iPad, iPod touch, or Mac.
5. Select or deselect each feature, then click Apply."

These steps may help. Let us know if you have further questions.

Take care.