When you are looking at various entries in the Azure AD logs you will find, under the Basic Info tab, a Sign-in error code and directly below that a Failure reason field as shown above.
The above, shows you these fields in more detail.
You may not be aware but if you navigate to the web site:
and plug in the Sign-in error code from the event, you should see information like that shown above. Most of it should match what the Failure reason field says. There can however, also be additional information in there that may help you when it comes to troubleshooting these events.