This error message will load if entering invalid "separators" between multiple email addresses in the Preliminary Copies line, located on the Communication tab within the Filings page.
If needing to list multiple email addresses for Preliminary Copies, please be sure to separate them with a comma.
As noted in this article, a Preliminary Copy is a notification email containing a copy of the file uploaded that is sent when the filer clicks on Submit, without having to wait on the court to accept the filing.