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

Starting from October 1, 2022, Microsoft will prohibit the use of plain-text username + password logins for all applications. Instead, applications should use the OAuth2 protocol for sign-in.

Please remove the "old connection" in the sync.blue® dashboard and create a new one. Use "Microsoft 365 - Exchange Online (Public contacts)" as the sync source and follow the instructions to be able to synchronize contacts using the new method.