The error usually occurs when the user triggers actions that require to send an email but outgoing email is not configured, but there are cases where the error appears on EWS configuration even after the outgoing emails are working fine.
1. Go to Admin > Mail Server Settings > Outgoing.
2. Uncheck, EWS (Exchange Web Services) and set SMTP / SMTPS with a working configuration.
3. Check if you can recreate the reported issues, if it works fine --- Confirm the Status by Adding notes and announcements.
4. Now, switch back to EWS (Exchange Web Services) and check the reported issue.