Exchange 2003 SP2 staged migration to O365

Hi.
Please check.
1. Your certificate must be created from Unified Communications certificate partners.
Unified Communications certificate partners
https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/message-trace-scc?view=o365-worldwide
2. Please test the connection from https://testconnectivity.microsoft.com/tests/exchange
3. Please check your step
Perform a staged migration of email in Exchange Online

https://docs.microsoft.com/en-us/exchange/mailbox-migration/perform-a-staged-migration/perform-a-staged-migration

4. Please check Basic Auth on your tenant.

UPDATE: Exchange Online deprecating Basic Authentication (Basic Auth)
https://docs.microsoft.com/en-us/lifecycle/announcements/exchange-online-basic-auth-deprecated

Example On-Premises Exchange 2003 to Microsoft 365 Migration Guide
https://help.bittitan.com/hc/en-us/articles/115008106507-On-Premises-Exchange-2003-to-Microsoft-365-Migration-Guide#exchange-server-2003-0-0