Sent messages should not be getting saved as drafts. Open Account Settings, Copies & Folders. What are the current settings for Sent messages and drafts? Your non-English screenshot won't help (me) much here, unfortunately.

I have the default settings.

Why saving the draft fails, I don't know. This may be a temporary problem with the email provider's IMAP server. Unless there's a compelling reason to save drafts in the Drafts folder on the server, I'd rather use the Drafts folder of the Local Folders account.