Using older versions of Outlook for Mac for personal email and calendar
This seems to be happening more and more for me, I'm on Outlook 16.88, but I have found a workaround that so far has worked every time. When this error comes up, I quit Outlook, re-launch, then click in the Inbox the email thread, and then on the Draft message in the thread I click 'Resume Editing' button on the right, then Send.