This site has exceeded its maximum file storage limit-SharePoint

Error Example:

Error while writing data to entity '<entityName>': More than 10 consecutive errors while writing the changes to the data entity. Write operation is aborted. The last error was 'Error while inserting new element: Error while uploading a file to '<filePath>': This site has exceeded its maximum file storage limit. To free up space, delete files you don't need and empty the recycle bin. SharePoint-Error code: 589923' Write operation is aborted​.

Cause:

You have exceeded the quota for storage on the site collection, which was configured when the site collection was created. The site then goes into read-only mode until the quota is increased or more room is made by deleting items. See these links for more information:

Manage site collection storage limits (Office Support)

Create, edit, and delete quota templates in SharePoint 2013 (TechNet)​

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SharePoint Storage Quotas in SharePoint Online

Resolution/Workaround:

As the error notes, you can free up space by deleting unnecessary files, records, lists/libraries, etc., and then empty the Recycle Bin for the site (may require that both the first-stage and second-stage bins be emptied). Alternatively, you can have the storage quota increased (which is explained in the links in the Cause section above).

There is an option to check what the quota is, how close to the quota the site is, and which sites/lists are taking up the most of the quota by going to Site Settings ->Storage Metrics.