The nuisance error, “The remote server returned an error: (429)” in Veeam Backup for Microsoft 365 indicates that you have exceeded the request rate limit imposed by Microsoft. This occurs when too many requests are made to Microsoft within a specific time frame. We commonly see this during the initial Veeam for M365 backup and the error resolves itself during the automatic retry, which is enabled by default.
If you would like to make changes on the backend in Microsoft, such as editing Retry-After headers or decorating traffic, you may refer to this Microsoft Article on how to avoid getting throttled in SharePoint.
Source: https://www.managecast.com/veeam-backup-for-microsoft-365-sharepoint-the-remote-server-returned-an-error-429/