Outlook refuses to send email "The operation cannot be performed because the message has changed"

Again and again when I want to send off an email Outlook (esp. when replying to a email) refuses to do so with the error message "The operation cannot be performed because the message has been changed".

What does this BS-message try to teach or tell me? And why does this precious piece of software have trouble sending out newly written emails. Isn't that part of its purpose of existence?