CU18 Attachment Problem. CU 18 Shared Mailbox Error.
The error will be that a shared Mailbox attachment can’t be opened in OWA. We’ve experienced a weird error if a user is opening a shared mailbox inside of OWA. Outlook clients report it to be opening fine. The error will only occur when you are opening an attachment linked to the email when using the “Open another mailbox” feature in OWA. This has been tested as a CU18 attachment problem in the latest update from MS Exchange.
However, browsing the shared mailbox works fine, the email attachment will try to download but you will get an error.
You’ll notice it will be trying to download from an incorrect URL such as: https://outlook.office365.com/owa/……..
As a workaround, don’t use the “Open another mailbox…” feature. Instead, right click on your root folder to get the menu. Then select “Add shared folder…”
After this you will find that you can open the attachments fine.
The original issue is that this is a bug from Microsoft’s CU 18 latest update: https://docs.microsoft.com/en-us/answers/questions/112628/exchange-2016-after-installation-of-cu-18-attachme.html