After a certain time a connector(Office 365 Users or Office 365 Outlook) will expire and will need to get fixed by the user. The error about the connector expiring will only show if the user logs in to Flow. This does not work if the user only sees the PowerApps side of things because they will not get any notification that their Flow failed because only the owners of the flow get it. An error that will send to the users would at least work but since the connection to Office 365 Outlook expires this is not possible either.
There should be a way for administrators to fix the users connection without the users having to going into Flow or remove the expiry for the connections all together. Adding a way for all users to just use one connection would work too because at least only one person will need to refresh the connection.