Have several users who are getting this error when sending message to multiple recipients
Error is [0x80070057-0x00000000-0x00000000].
Failed to upload attachments to server (error 0x80070057).
Submit-Message failed: message id(17), failure enum(29), HResult(0x80070057), EC(0).
Also seeing the errors when opening a message that message has been changes. Do you want to keep changes?
Thirdly, some users cannot forward meetings to users. they get: "! Unknown Error." popup. This is for internal and external meetings.
All of this works in OWA, so I believe these to be Outlook issues.
Perhaps a recent update is causing this?