MS has a desktop tool you can download from your O365 account under tools - however if you are running Office 2013 it will report that you don't need to run it. Try removing those and see if the issue persists - if it goes away, try re-adding them one at a time It may be they just needed to be re-added after some action occurred, or it maybe permissions have changed on one of those items. Fill out the information needed, including the name that you’d like to appear when sending emails, new email address, and password, then click Next. Click New: The Auto Account Setup screen will appear. Other things to look for are if you are connecting to other calendars (Exchange or SharePoint). Open Outlook 2013, then click on File and choose Add Account. To Manually configure your connection you search for that phrase directly and you'll get good results. I recommend manually configuring your client and notify your Admins (if you are the admin - review all your DNS settings). If you are showing legacy connections (anything not O365) your organization may have issues with how they have configured Auto Discover. Verify all your connections are pointing to O365 and not a previous/legacy Exchange server (look at server names - they should all be ). In your task bar notification area find the outlook icon CTRL+Right Click and select 'Connection Status'. When Outlook is running check it's connections.Make sure your office installation is fully updated.

There are quite literally a dozen reasons this can happen.

Outlook 365 exchange 2013