Outlook Classic Freeze & PST Corruption (KB5074109)
The Problem: The January 2026 Patch Tuesday update (KB5074109) causes Classic Outlook to freeze with “Not Responding” errors, primarily affecting users with PST files stored on OneDrive. Symptoms include:
- Outlook fails to open or reopen without Task Manager intervention
- Sent emails disappear from Sent Items (though actually sent)
- Continuous redownload of same emails
- Error loop requiring process termination
Expert Fix (Not Found on Standard Support Sites):
powershell
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# Step 1: Isolate PST from OneDrive sync immediately
# Move PST to non-synced location via symbolic link
mklink "C:\OutlookData\Archive.pst" "D:\LocalOutlook\Archive.pst"
# Step 2: Reset Outlook navigation pane (fixes ghost entries)
outlook.exe /resetnavpane
# Step 3: Disable Modern Authentication cache corruption
# Navigate to: File > Office Account > Account Privacy > Manage Settings
# Turn OFF "Enable optional connected experiences"Pro Tip: Create a batch file to automate PST backup before updates:
batch
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@echo off
xcopy "C:\Users\%username%\Documents\Outlook Files\*.pst" "D:\PST_Backup\%date:~-4,4%%date:~-10,2%%date:~-7,2%\" /I /Y