What does "Invalid credentials" or "Authentication error" mean in the context of Exchange/EWS and Microsoft 365?

Possible cause: Access data not correct

Please check first if username and password are really stored correctly in sync.blue®. Check if a login with these credentials via https://outlook.office365.com is possible.

Possible cause: Default authentication deactivated

Microsoft prohibits from 1.10.2022 for all applications the login via username + password in clear text. Instead, applications should perform a login via the OAuth2 protocol.

sync.blue® will soon provide modern ways to securely synchronize contacts via OAuth2 (EWS/Exchange) with Microsoft 365 and will inform about it via mail.