Save as draft
With Thunderbird 78.3.3 (64-BIT) I get the error message : "Save as draft failed. Your message could not be saved as draft. Send message failed."
Alle antwurden (3)
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.