@Anthony Ramsay Welcome to the Atlassian Community. There happened to have been an outage on Cloud earlier today that may have affected this editing. Can you confirm whether this is still an open issue for you? Have you refreshed the page and are you still getting the same error?
One other thing to look into - do you already have a page in this space with the same Title? Titles are unique to a space and this may be preventing you from saving the page with the title you are trying to use.
Let us know so we can assist.
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