Based on your description, I have a couple questions I need to check with you:
1.Is Andrea's email address Online or On-prem?
2.Is the error only reported when going through a browser and third party apps? Does the problem occur with Outlook as well?
“Microsoft.Exchange.Data.Storage.MailboxOfflineException” This is a more specific error that indicates a problem with accessing a mailbox on an Exchange server, possibly because the mailbox is offline or there is a connectivity issue with the Exchange server.
I need more information from you to further resolve the issue.