Hello there, I had the same problem this morning with one of our colleagues.
Now I'm not sure if any of you are trying to add files through the same way that we were doing, but I managed to solve it in this instance.
What I did was grant Outlook full access to the files on the disk. You apparently need to do that under system preferences > security and privacy > full disk access.
I added Outlook to this list, restarted Outlook and then reconnected to our server using the Command-K shortcut.
After that, I was able to add attachments to any new mails I created without it giving the very non-specific "attachment couldn't be added" error message.
Hope this'll help in your reported cases as well.