Office error "We can't sign you in" – the quick fix

Yeah, that "We can't sign you in" popup in Office is infuriating. You enter your credentials, it spins, then dumps you right back at the login screen. Been there. Here's the fix that works 9 times out of 10.

The fix: clear cached credentials via Credential Manager

Windows stores your Office login tokens in a place called Credential Manager. When those tokens get corrupted or stale, Office refuses to authenticate. The fix is wiping them clean.

1. Close all Office apps — Word, Excel, Outlook, the works.
2. Open Control Panel. Easiest way: hit the Start button and type "Control Panel".
3. Click User Accounts, then Credential Manager.
4. Select Windows Credentials.
5. Scroll through the list until you find entries that start with MicrosoftOffice or Microsoft.AAD.
6. Click each one and choose Remove. There might be 2-5 entries. Remove them all.
7. Restart your machine. Don't skip this — it flushes leftover cache.
8. Open any Office app. You'll be prompted to sign in again. Use your work or school account credentials.

That's it. You're back in.

Why this works

Office 365 uses OAuth 2.0 tokens stored locally. When they expire or get corrupted, the authentication handshake fails. Credential Manager holds these tokens. Removing them forces Office to request fresh ones from Azure AD. No reinstall, no registry edits, no prayers to the IT gods.

This issue hits hardest after a password reset, a domain join change, or a Windows update that tweaks authentication protocols. I've seen it on Windows 10 22H2 and Windows 11 23H2 equally.

Less common variations

1. Old Office 2016 or 2019 (non-subscription)

If you're running a perpetual license, the error might stem from a product key mismatch. Run cscript "%ProgramFiles%\Microsoft Office\Office16\OSPP.VBS" /dstatus in an admin command prompt. Look at the last 5 digits of the installed key. If it doesn't match your license, run the repair from Settings > Apps > Microsoft Office > Modify.

2. Shared computer activation

In RDS or VDI environments, shared computer activation can block sign-ins. Check HKLM\SOFTWARE\Microsoft\Office\16.0\Common\Identity\EnableADAL. If it's set to 0, change it to 1. Also verify DisableADALatopWAMOverride isn't set to 1. Both of these disable modern auth — you want them off.

3. Proxy or VPN interference

Some corporate proxies strip authentication headers. Test by temporarily disabling your VPN or proxy. If it works after that, add login.microsoftonline.com and *.office.com to your proxy's bypass list. Talk to your network team about that.

4. Corrupt Office profile

Rare, but happens. If clearing credentials didn't work, run the Microsoft Support and Recovery Assistant (SaRA) from https://aka.ms/SaRA-OfficeSignIn. It automates the credential wipe and adds a few registry checks. I've used it maybe 5 times in my career — it's a hail mary, but it works when nothing else does.

Preventing this from happening again

• Don't use saved passwords in browsers for Office 365. The browser's credential store can conflict with Office's own cache. Use a password manager instead.
• Keep Windows and Office fully updated. Microsoft has patched several auth bugs over the years. KB5006670 from 2022 specifically fixed a sign-in loop on Windows 10.
• If you're on a shared PC, enable shared computer activation. It handles token refresh properly. Google "Set-AADSharedComputerActivation" for the PowerShell script.
• After a password reset, reboot before opening Office. That clears any lingering tokens in memory.

That's the whole thing. Took me maybe 3 minutes to type. Should take you less to fix. Good luck.