The server runs a backup job at regular intervals. While this backup job is running the site needs to be set to read only mode. During this time you won’t be able to create or reply to topics or Private Messages. The job only takes a few minutes to run, but if you happen to be composing something and want to send it then you may see a message like this:
If you see this message, don’t panic. Just wait a few minutes then try again.
While the backup job is running the system also puts a notification banner across the top of the page. It looks like this: