outlook won't open

If KB5074109 is installed and the affected Outlook account uses POP, install KB5078127 before changing the Outlook profile or add-ins.

1. Check the post-update fix first

This specific update issue applies only when Windows Update history shows KB5074109 and classic Outlook uses a POP account.

1. Open Settings → Windows Update → Update history.
2. Look for KB5074109.
3. If it is present, check whether KB5078127 is also installed.
4. If KB5078127 is missing:
   • First use Settings → Windows Update → Check for updates.
   • If it is not offered, download KB5078127 from the Microsoft Update Catalog.
5. Restart Windows after installing that update, then test classic Outlook.

KB5078127 is the reported fix for classic Outlook POP profiles hanging at Loading Profile after KB5074109.^1^

If KB5074109 is absent, the account is Exchange/Microsoft 365 or IMAP, or KB5078127 does not fix it, continue below.

2. Test Outlook without add-ins

Since restarting Windows and repairing Office did not help, do not repeat those procedures.

1. Press Windows key + R.
2. Enter:

      outlook.exe /safe
   

3. Select OK, accept the default profile, and sign in if requested.

If Outlook opens in Safe Mode:

1. Select File → Options → Add-ins.
2. At the bottom, select COM Add-ins → Go.
3. Record or screenshot the enabled add-ins.
4. Clear their check boxes—disable them; do not select Remove.
5. Close and start Outlook normally.
6. Re-enable add-ins individually, restarting Outlook after each one, until the hang returns. Leave the responsible add-in disabled.

Safe Mode starts Outlook without its add-ins, which isolates whether one is blocking startup.^2^

3. If Safe Mode also hangs, test a new Outlook profile

Creating a profile does not require deleting the current one:

1. Hold Shift while starting Outlook. Alternatively, press Windows key + R and enter:

      outlook.exe /profiles
   

2. In the profile picker, select Options → New.
3. Give the profile a new name and add the email account.
4. Select the new profile and start Outlook.

If the new profile opens, the original Outlook profile is damaged. Keep the old profile until any locally stored data has been verified or copied; do not remove it as part of this test.^3^

About the “Failed to fetch / StartupData” details

Those details reference an Outlook web script (owamail) and are not, by themselves, proof of what is causing the separate classic Outlook Loading Profile hang.

• Open the same mailbox in Outlook on the web using another browser or device.
• Check Microsoft service health.
• If the same StartupData failure follows the account across browsers or devices, treat it as a mailbox/service issue rather than continuing local Office repairs. For a work or school account, the Microsoft 365 administrator should verify the Exchange Online license and Outlook-on-the-web access, then open a support request if only that mailbox remains affected.^4^

If further diagnosis is needed, provide the Windows build, the KB numbers shown in Update history, and whether the account is POP, IMAP, or Exchange/Microsoft 365.

References

1. Outlook hanging and now hangs at loading profile. - Microsoft Q&A
2. I can’t start Microsoft Outlook or receive the error “Cannot start ...
3. Create a profile in classic Outlook for Windows
4. Outlook on the Web startup failure – HTTP 500 (StartupData)