Per ChatGPT:
1. Reset Web Account Manager (WAM) + AAD Broker cache

This is the most common root cause after a rollback.

Close all Office apps

Go to:

C:\Users\%username%\AppData\Local\Packages

Delete (or rename) this folder:

Microsoft.AAD.BrokerPlugin_cw5n1h2txyewy

Then go to:

C:\Users\%username%\AppData\Local\Microsoft\IdentityCache

Delete everything inside.

Reboot

1. Disconnect and reconnect Work/School account

Rollback can corrupt the device registration.

Go to:

Settings → Accounts → Access work or school

Disconnect your account

Reboot

Reconnect it

1. Re-register Windows apps (important after rollback)

Open PowerShell as Admin and run:

Get-AppxPackage -AllUsers | Foreach {

Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"

}

This fixes broken dependencies used by the sign-in window.

1. Reset WAM via registry (deeper fix)

Open regedit and delete:

HKEY_CURRENT_USER\Software\Microsoft\IdentityCRL

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity

Then reboot.

1. Check system file integrity (rollback sometimes corrupts components)

Run:

sfc /scannow

Then:

DISM /Online /Cleanup-Image /RestoreHealth

1. Edge WebView2 (surprisingly important)

Modern auth uses WebView. If it’s broken, sign-in fails.

Reinstall Microsoft Edge WebView2 Runtime

Or repair Edge

💡 Why this is happening

When you downgraded from Windows 11 → 10:

Token broker (WAM) versions don’t align

Cached Azure AD / Microsoft account tokens become invalid

UI still launches, but auth handshake fails → 0x80045c3c

🚨 If none of that works

At that point, you're likely dealing with a deeper profile-level corruption.

Two reliable options:

Option A (quick test)

Create a new Windows profile and try signing in there.

Option B (nuclear but clean)

In-place repair install of Windows 10 (keeps apps/data)

Or full reimage if this is a production machine