I would think that it might be a user profile issue (either Outlook profile or Windows profile). Try removing the profile and see if that helps (first try the Outlook profile then the Windows profile for the user).
NOTE: Before you remove the Windows profile, make sure to back up the files first (the Windows profile hold user documents, Favorites and lots of other stuff).
Also, you may want to try and see if it will connect when you manually configure the Outlook profile for User1 on Computer21 (versus using Autodiscover).
I'm not sure if the problem could have been solved in another way but I am going to present you what finally worked for me. It seemed that the user's mailbox was damaged somehow. I still don't know exactly what the problem was. So my solution finally was to re-create the users's mailbox.
Export the users's email to a pst-file using the Exchange Management Shell:
New-MailboxExportRequest -Mailbox "username" -FilePath "\\path-to-share\filename.pst"
You can check the status using:
Get-MailboxExportRequest | Get-MailboxExportStatistics | fl
It may happen that either the ExportRequest or the ImportRequest gets stuck in the status "Queued". In my case I had some other ExportRequests in my Get-MailboxExportRequest result. You may remove the completed requests using this command per example:
Get-MailboxExportRequest -Status Completed | Remove-MailboxExportRequest
or a specific one using this:
Remove-MailboxExportRequest -Identity "OU\structure\to\useraccount\MailboxExport1"
After I removed all other requests the status of the desired request changed from "Queued" to "In Progress".
Once the export is completed we need to disable the user's mailbox first. Be aware that removing the mailbox before it was disabled would also remove the whole active directory user account. The disabling removes the connection between the user account and the mailbox.
Disable-Mailbox -Identity "username"
Now we need the MailboxGuid of the disabled mailbox. We can list the disabled mailboxes on our exchange database using:
Get-MailboxStatistics -Database “Mailbox Database Name" | where {$_.disconnectdate -ne $null} | select displayname,MailboxGUID
In my case (Microsoft Exchange Server 2013 CU4 (SP1)) the disabled mailbox wasn't in the list so I listed all other mailboxes which in my case is still OK because I don't have that many. Other users may need to use a more specific filter in their command:
Get-MailboxStatistics -Database “Mailbox Database Name" | where {$_.disconnectdate -eq $null} | select displayname,MailboxGUID
However I found the user's mailbox and copied the MalboxGuid. Then I removed it using:
Remove-Mailbox -Database “Mailbox Database Name" -StoreMailboxIdentity 92d20afd-42d8-496e-9455-34b3d6cb066e
The user's mailbox is now deleted and we are ready to create a new one. I simply logged into "ECP" and created a new mailbox for the user. After the Mailbox got created we are ready to import the exported emails into the new mailbox using:
New-MailboxImportRequest -Mailbox "username" -FilePath "\\path-to-share\filename.pst"
As before the status of this procedure can be checked using:
Get-MailboxImportRequest | Get-MailboxImportStatistics | fl
After the Import is finished I would recommend to delete the user's outlook profile and create a new one. In my case it was still buggy before I did this, and I also removed and re-assigned the permissions to other mailboxes for the user, just in case.
One last thing: After the whole procedure it happened that internal users who tried to send emails to the user's email account got an "email could not be delivered" error. I think this is because the MailboxGuid of course has changed and the server still tries to deliver the emails to the old mailbox. The users who are trying to send an email to the user's mailbox needed to get the latest changes in their offline address book done. It doesn't seem to affect all users but some, so I created a tutorial for the affected users and sent it to them if they reported the error.
I hope this will help someone who's also unlucky to have the same problem.
Best Answer
This is a known issue. From Office 365:
Exchange Online service alert
Title: Issue affecting viewing email content in Outlook
ID: EX255650
Status:
Restoring Service
Title:
Issue affecting viewing email content in Outlook
User Impact:
Users may be unable to view email message content within Outlook. More info: Impact is specific to the Outlook client and users with access to other protocols, such as Outlook on the web or the Outlook mobile app, can view message content in those platforms as a potential workaround while our fix is applied.
Current status:
We’ve identified the underlying cause of impact and are applying a fix. This fix will reach all affected users incrementally over the course of the next four-to-five hours. Once users receive the fix, they will need to restart their email client to apply the fix. In some circumstances, users may need to restart their client a second time for the changes to take effect. We expect to complete this process and restore service for all affected users by May 12, 2021, at 3:00 AM UTC. We encourage affected users who are able to do so to leverage the workarounds described above in the "more info" section of this post while we complete the process of fixing this problem. Scope of impact: This issue could affect any user attempting to view an email message in the Outlook client.
Root cause:
A recent change to systems that facilitate text display management for content within the Outlook client caused impact.
Next update by:
Wednesday, May 12, 2021, at 3:00 AM UTC
FYI, Office 365 service alerts are emailed to all tenant admins. Make sure you have a tenant admin that can receive email so that you get these alerts.
You can also check Office 365 service health at the URL below:
https://admin.microsoft.com/adminportal/home?#/servicehealth