Fixing “Sign-in required” loops when activating Python cells.

November 2, 2026 6 min read

When entering a =PY() formula or recalculating a Python-enabled workbook, users frequently become trapped in an authentication loop. Excel displays a banner or modal stating: Sign in required or Please sign in with your organizational account to use Python in Excel.

Entering valid credentials appears to succeed, but seconds later the prompt reappears, the formula bar locks, and Python cells revert to #PYTHON! with a status card indicating that the user is unauthenticated.

This loop occurs because Python in Excel requires an active, validated Microsoft Entra ID (Azure AD) user token to authorize calls to the Microsoft Cloud container service. When cached identity tokens in Windows become corrupted, when multiple conflicting Microsoft accounts are signed into Office simultaneously, or when conditional access policies invalidate background token refreshes, the authentication handshake fails repeatedly.

Quick Fix: Clear Corrupted Office Identity Tokens

Sign out of all accounts in Excel via File > Account > Sign Out. Close all Office apps, open Windows Credential Manager, and delete all entries under Generic Credentials starting with MicrosoftOffice16_Data or MS.Outlook. Open File Explorer, navigate to %localappdata%\Microsoft\IdentityCache, and delete all contents. Relaunch Excel and sign back in using your primary licensed Microsoft 365 organizational account.

Why Python Cells Trigger Sign-In Loops

The architecture of Python in Excel connects local formula execution to cloud compute resources. Every time a Python cell calculates, Excel’s internal Web Account Manager (WAM) checks for a valid OAuth bearer token:

[Local Excel Worksheet: =PY()]
               │
               ▼
   [Web Account Manager (WAM)]
               │
               ├── Valid Cached Bearer Token ──► Connects to Cloud Container Runtime
               │
               └── Corrupted / Expired Token
                       │
                       ▼
          [Interactive Sign-In Prompt]
                       │
          (User Enters Credentials)
                       │
          ┌────────────┴────────────┐
          ▼                         ▼
   [Token Cached Cleanly]     [Identity Conflict / WAM Lockout]
          │                         │
     Runs Python               Loop: Re-prompts on next calculation cycle
  1. Cached Token Corruption: Windows caches authentication tokens locally in the Web Account Manager (WAM) and Credential Manager. If a token expires while an asynchronous container session is active, or if a password was recently changed, Excel may continuously attempt to use the invalid cached token instead of completing a fresh authentication challenge.
  2. Multi-Account Profile Collision: Many users have both a corporate account (user@company.com) and a personal Microsoft account (user@outlook.com) signed into the same Office installation. If Excel defaults to using the personal profile for cloud connections, the Python service rejects the token because Python in Excel is an enterprise-governed feature.
  3. Conditional Access and MFA Session Limits: Corporate IT security policies frequently enforce Conditional Access rules (e.g., re-authenticating every 8 hours, requiring compliant devices, or enforcing location boundaries). If the cloud container service requests a background token refresh that fails an interactive MFA requirement, Excel halts calculation and prompts the user to sign in repeatedly.
  4. Proxy and Network Token Drops: If your network proxy intercepts the authentication handshake and strips bearer headers, the cloud container drops the connection immediately, as covered in Troubleshooting “Proxy Server” authentication failures for Python calls..

Diagnostic Workflow: Identifying the Authentication Bottleneck

Follow this sequence to trace why your user credentials are not sticking:

Step 1: Check the Active Connected Service Account

  1. Open Excel and click File > Account.
  2. Look at the left pane under User Information.
  3. Under Connected Services, check which account is listed.
    • If a personal OneDrive or external account is listed as the primary connected service, Excel is sending the wrong identity token to the Azure container.
    • Your commercial work or school account must be the primary identity.

Step 2: Check for Yellow Warning Banners

Inspect the top right corner of the Excel window near your user profile name:

  • If a yellow triangle with an exclamation mark appears over your avatar, Office has experienced an account synchronization failure across all applications (Word, Excel, Outlook).
  • Click the profile icon and select Fix Account. If the prompt fails to resolve the warning, your Windows identity cache is locked and must be cleared manually.

For related file-level authentication blocks on cloud files, see How to fix “Upload Blocked” and “Sign in required” for OneDrive/SharePoint files..

Step-by-Step Remediation Strategies

Strategy 1: Clear Cached Office Identity and Credentials (Most Effective)

Clearing old authentication tokens forces Windows to generate a clean, uncorrupted security context:

  1. Close all Microsoft Office applications (Excel, Word, Outlook, Teams).
  2. Press Ctrl + Shift + Esc to open Task Manager. Ensure no orphaned EXCEL.EXE background processes are running.
  3. Press Windows Key + R, type control keymgr.dll, and press Enter to open Credential Manager.
  4. Click on Windows Credentials.
  5. Scroll down to Generic Credentials.
  6. Look for entries starting with:
    • MicrosoftOffice16_Data:...
    • MS.Outlook:...
    • ADAL:...
  7. Click the drop-down arrow next to each Office-related credential and click Remove.
  8. Press Windows Key + R, paste the following path, and press Enter:
    %localappdata%\Microsoft\IdentityCache
  9. Delete all files and folders located inside the IdentityCache directory.
  10. Relaunch Excel, go to File > Account, and sign back in with your licensed corporate credentials.

Strategy 2: Remove Competing Microsoft Accounts in Windows

If Windows is silently authenticating with a secondary, unlicensed personal account:

  1. Open the Windows Settings app (Windows Key + I).
  2. Navigate to Accounts > Access work or school.
  3. Check the accounts listed:
    • If an old, stale corporate account or secondary personal account is present, click on it and select Disconnect.
  4. Go to Accounts > Email & accounts.
    • Under “Accounts used by other apps,” ensure your primary business account is selected. If a personal account is set to “All apps can sign in,” change it to “Microsoft apps must ask to use this account.”
  5. Restart your workstation.

Strategy 3: Repair the Web Account Manager (WAM) Broker Plugin

In Windows 10 and Windows 11, Office uses the modern Web Account Manager (WAM) framework for authentication. If the WAM broker package becomes unregistered or corrupted:

  1. Right-click the Windows Start menu and select Terminal (Admin) or PowerShell (Admin).
  2. Run the following command to re-register the default Microsoft AAD broker plugin: if (-not (Get-AppxPackage Microsoft.AAD.BrokerPlugin)) { Add-AppxPackage -Register "$env:windir\SystemApps\Microsoft.AAD.BrokerPlugin_cw5n1h2txyewy\Appxmanifest.xml" -DisableDevelopmentMode -ForceApplicationShutdown } Get-AppxPackage Microsoft.AAD.BrokerPlugin
  3. Restart your computer. This repairs broken authentication broker dialogs that prevent sign-in tokens from passing back to Excel.

Strategy 4: Verify License Status and Tenant Entitlements

If the sign-in loop persists despite clean credentials, the cloud container service may be actively rejecting your identity due to a missing service plan:

  1. Open a browser and sign in to portal.office.com/account.
  2. Click on Subscriptions.
  3. Under your active license (e.g., Microsoft 365 E5 or Microsoft 365 Business Premium), confirm that your account includes access to modern cloud features.
  4. If you belong to an external organization accessing a shared sheet, refer to the permission rules in Troubleshooting “Multi-tenant Access” errors with Python workbooks.. For licensing plan details, consult Solving “Subscription Required”: Why Python in Excel is disabled in your tenant..

Verifying Sign-In Stability

To verify that the authentication loop is broken:

  1. Run a Calculation Test: Open Excel, create a new workbook, and enter =PY("10 * 10"). The cell should transition from “Pending” to 100 without triggering authentication dialogs.
  2. Test Persistence Across Restarts: Save the workbook, close Excel completely, and reopen the file. Recalculate the sheet (Ctrl + Alt + F9). The formula should compute immediately without displaying “Sign in required” banners.
  3. Inspect Connectivity: If the sign-in prompt is replaced by a connection drop, check your network path using Fixing “Connectivity Error”: When Excel cannot reach the Microsoft Cloud container..

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