Fix 0XC000041C: Invalid User Principal Name on Windows

Yeah, this one's a pain. You type your password, wait, and then Windows throws error 0XC000041C right in your face. It says your User Principal Name is invalid. Don't panic. It's a mismatch between what Windows thinks your UPN is and what the domain actually has.

Quick Fix: Update the UPN and Clear Cached Credentials

This fix works for most cases. It takes about 10 minutes.

Open Active Directory Users and Computers on your domain controller. If you don't have access, ask your admin. Right-click your user account and choose Properties.
Go to the Account tab. Look for User logon name. That's your UPN. Make sure it matches the email format you're using to log in. For example, if you log in as jdoe@contoso.com, the UPN should be jdoe@contoso.com, not jdoe@corp.contoso.com.
If it's wrong, correct it. Click Apply then OK. After this, you should see the change reflected immediately.
On the affected machine, open Command Prompt as administrator. Type klist purge and press Enter. This clears cached Kerberos tickets. You should see confirmation that tickets were purged.
Now, clear Windows credential cache. Open Control Panel > User Accounts > Credential Manager. Click Windows Credentials. Look for anything related to your domain or the server you're connecting to. Remove those entries. After removing them, reboot.
Log in again using the correct UPN. It should work now. If it doesn't, try dsregcmd /leave in Command Prompt, then dsregcmd /join to rejoin the device to Azure AD if you're using hybrid.

Why This Happens

The UPN is like a license plate for your account on the network. If the plate has a typo or is old, Windows can't prove who you are. This error usually pops up in two scenarios:

You changed your UPN in Active Directory but didn't update the local machine's cached info. Windows still thinks you're the old UPN.
You're trying to log into a server or service that uses a different UPN suffix than what you normally use. For instance, your computer is on oldcompany.com but your account is now on newcompany.com after a merger.

Less Common Causes and Their Fixes

UPN Suffix Not Added to Active Directory

Sometimes your admin has to add the new UPN suffix first. On your domain controller, open Active Directory Domains and Trusts, right-click the top node, choose Properties, then add the suffix under UPN suffixes. Without this, Windows rejects the UPN outright.

Azure AD Sync Issues

If you're in a hybrid environment and the UPN changed in Azure but not on-prem, or vice versa, you get this error. Run Get-MsolUser -UserPrincipalName jdoe@contoso.com | fl UserPrincipalName, ImmutableId in PowerShell to check. If they don't match, update the UPN in Azure AD using Set-MsolUserPrincipalName.

Corrupt Credential Manager Database

On rare occasions, the Vault folder gets corrupt. Close all apps, kill explorer.exe, then delete the contents of C:\Users\[YourUsername]\AppData\Local\Microsoft\Vault. Restart explorer. This nukes the vault but also fixes stubborn credential issues.

Pro tip: Before you go deleting vaults, test logging in with a different local account. If that works, it's your profile's vault, not the domain.

How to Prevent This in the Future

Always update UPN changes on the domain controller first. Wait 15 minutes for replication before logging off.
If you rename a user's UPN, send them a script to run klist purge and reboot. Or use Group Policy to push a logoff script that does it.
Document UPN suffixes for your org. If someone adds a new suffix but forgets to add it to AD Domains and Trusts, you'll get this error.
Check Azure AD Connect settings if you use hybrid. Make sure the UPN attribute is mapped correctly. A misconfiguration there causes this error daily for users.

The real fix is making sure the UPN is consistent across all systems. Once you sync it, the error goes away and stays away.