In this post I point you to a very helpful admin resource hub which offers you an entry point in case your are facing issues with Microsoft Office 365. There you’ll find information on troubleshooting for …

• Office 365,
• Office,
• SharePoint,
• Exchange,
• Outlook,
• Teams and
• Skype for Business.

Conclusion, opinion and summary

In my experience the most common issues are related to things like

• Connectivity from/to Office 365 services,
  e.g. a client cannot connect to certain IPs/URLs to use a certain service successfully and completely (!) etc.
• Not all prerequisites are satisfied, e.g. UPNs are not ok, e.g. userfirst.userlast@companydomain.local or else.
• Misconfiguration/s of clients, e.g. GPOs to redirect an Outlook client always to an onpremise Exchange, not updated proxy pac files etc.

Before you start investigating you should always start to to ask some common questions, for example:

• How many users are affected?
  E.g. does it affect a single user or all users at a certain branch site …
• Where are you (and the affected users) located?
  E.g. Home Office, corporate premises …
• What’s the issue, could you please describe the issue in more detail?
  E.g. my Outlook client asks several times a day for credentials …
• Did you try to connect from another network?
  E.g. does the issue also occur if you use a network cable instead of WiFi? Or did you experience the issue only if you are working remotely from home?
• Did you try this from another computer?
• …

In case you check all this you can then dig deeper into the related client or service to figure out what the (root) cause is and how to resolve it. For that, you might go to the related “Office troubleshooting for admins and IT professionals” page on Microsoft docs to proceed troubleshooting and issue resolution.

Below, I also added two helping tools, the well-known Remote Connectivity Analyzer and the Network Testing Companion. The first one is an online troubleshooting analyzer and the second is a PowerShell-based tool which enables you to check client connectivity towards Office 365, either directly within PowerShell or you can open the Network Testing Companion GUI (by executing Invoke-NetworkTestingClient in PowerShell).

Additional resources

• Office troubleshooting for admins and IT professionals
• Microsoft Remote Connectivity Analyzer
• Microsoft Network Testing Companion