Outlook error 'An extension file failed to initialize' (extend.dat)

windows

Symptoms

After installing ActivClient, Outlook throws an extend.dat / extension-file error on startup.

Likely cause

Corrupt extend.dat caching after the install.

Fix

1. Close Outlook. 2. Show hidden files (Control Panel > Folder Options > View > Show hidden files). 3. Navigate to: - Vista/7/10/11: C:\Users\<user>\AppData\Local\Microsoft\Outlook - XP: C:\Documents and Settings\<user>\Local Settings\Application Data\Microsoft\Outlook 4. Rename extend.dat to extend.bak. 5. Reopen Outlook (it will rebuild the file).

Sourced from militarycac.com. Rewritten for clarity.

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