The classic Outlook app for Windows was working fine, except it did not one day when I opened it. Within seconds of opening, it would freeze with the window title showing Outlook (Not Responding).
I used the support article provided by Microsoft to try various solutions:
Safe mode: Outlook can be opened in safe mode by launching outlook.exe /safe. It worked fine in safe mode!
Disable all unnecessary addins: I disabled them but Outlook still froze when opened in normal mode.
Repair inbox: The scanpst tool located at C:\Program Files\Microsoft Office\root\Office16\SCANPST.EXE can be used to detect and fix errors in the PST file. Running it did find some errors, I requested it to repair them and it did that. However, this did not solve the Outlook freezing error.
None of the solutions worked, so I decided to click the New Outlook option at the top-right corner as soon as I opened the classic Outlook. It switched permanently to the new app and that worked fine.