Hello,
Usually this error occurs because the Content Database is full, most times because Sharepoint has been installed over SQL Express edition which limits the size of the Databases to 4 gigabytes.
For the rest, this error message could be related to throttling: https://learn.microsoft.com/en-us/sharepoint/dev/general-development/how-to-avoid-getting-throttled-or-blocked-in-sharepoint-online#what-is-throttling
SharePoint Online uses throttling to maintain optimal performance and reliability of the SharePoint Online service. Throttling limits the number of user actions or concurrent calls (by script or code) to prevent overuse of resources.
That said, it is rare for a user to get throttled in SharePoint Online. The service is robust, and it is designed to handle high volume. If you do get throttled, 99% of the time it is because of custom code. That doesn't mean that there aren't other ways to get throttled, just that they're less common. For example, you spin up 10 machines and have a sync client going on all 10. On each sync 1 TB of content. This would likely get you throttled.
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