In our primary sandbox environment we had an issue with the main Community page. Whenever the page was loaded in the browser, a prompt would appear reading "Try reloading the page or try again later". The user could clear the issue by clicking an X on the prompt, and everything else worked normally. This was not happening in Production nor in other sandboxes created from Production.
SFDC fixed this issue and it seemed to be a known issue for them, so I'm posting this here for future users who have this issue. Answer below.