Apple Mail says Unable to Attach

As part of my work I regularly send email attachments to clients. As of today, nothing will attach from Mail on my MacSudio running Sequoia 15.4.1. The attachments are no more than 1 or 2MB. As of today, anything I try to attach gives me the message "Unable to attach." (See screenshot) This happens to any and all attachments: pdfs, pngs, jpegs.. If I go into Mail on iCloud, I can do the attachments, but this is very tedious. I have restarted and all those obvious things. Any ideas?

[Edited by Moderator]