SharePoint Online or OneDrive For Business returns HTTP status code 429 ("Too many requests") when you are migrating to SharePoint Online or OneDrive For Business using the endpoint type SharePoint or OneDrive For Business. These endpoints use the client-side object model (CSOM) API, which is subjected to throttling from Microsoft. This can result in errors when MigrationWiz exceeds the CSOM API usage limits.
When CSOM API throttling limits are continually exceeded over a long period of time, the SharePoint Online CSOM API will throttle any further requests from MigrationWiz in order to prevent overuse of SharePoint or OneDrive For Business resources. Once the throttle is in effect and a 429 error is returned, MigrationWiz will use an incremental back-off in order to reduce the number and frequency of calls. However, in some cases, MigrationWiz can continue to exceed those usage limits. This results in SharePoint Online completely blocking the migration and returning an HTTP status code 503 ("Service unavailable").
If this occurs:
- Stop the migration for each item. KB004662
- Wait for a period of 24 hours.
- Resubmit each item for migration. Migrations will begin from where they left off, at the time of the stop. KB005332
Note: You really do have to wait for 24 hours before resubmitting your migration.