Microsoft 365 apps are sluggish, show a blank window or fail to sync.
1. Fully quit the app (right-click the tray icon → Quit). 2. Teams: delete the cache folder %appdata%\Microsoft\Teams (classic) or %localappdata%\Packages\MSTeams_8wekyb3d8bbwe\LocalCache (new Teams). Outlook: File → Account Settings → Data Files → Open location → rename the .ost file while Outlook is closed. 3. Restart the PC and open the app; it rebuilds the cache (may take a few minutes). 4. If it persists, submit a request with a screenshot of the error.