Outlook 2019 on Windows 10 closes by itself about 5 minutes after I open it. No error, it just disappears. I reinstalled Office and it did not help.

A guy on another forum said I should try a clean boot to see if some other program is the problem. I have no idea what that means. Is it safe and how do I do it?

A clean boot just starts Windows with only Microsoft services and no third party startup programs. It does not delete anything and you can undo it in a minute. The point is to find out whether a background program (antivirus, backup tool, a sync client) is clashing with Outlook.

Steps for Windows 10:

1. Press Win + R, type msconfig and press Enter.
2. On the Services tab tick "Hide all Microsoft services" first. This step matters, do not skip it.
3. Click "Disable all".
4. On the Startup tab click "Open Task Manager", then disable every item in the list there.
5. Close Task Manager, click OK in System Configuration and restart.

Open Outlook and use it for a while. If it stops crashing, one of the items you disabled is the cause. Turn them back on in small groups, restarting each time, until the crash returns.

Did it. Outlook has been open for 40 minutes and no crash. So it is one of my startup items. How do I get back to normal startup without redoing everything by hand?

Go back to msconfig. On the General tab choose "Normal startup" and it re-enables everything for you after a restart.

To find the guilty program faster, do it in halves. Enable half the services, restart, test. Whichever half brings the crash back has the problem. Repeat on that half. Usually it takes 4 or 5 restarts.

Also try this separately, because it takes 10 seconds: press Win + R and run outlook.exe /safe. Safe mode loads Outlook with add-ins disabled. If it is stable there, go to File > Options > Add-ins > Manage COM Add-ins and turn them off one by one. A PDF or CRM add-in is a very common cause.

Found it. It was a PDF add-in that came with an old PDF tool. Disabled it and Outlook has been fine all day. Thanks both.