Can you clone the email and edit the copy?
Have you cleared your cache? Have you tried accessing edit draft in the incognito/private browser window? I'd also recommend logging out of your Marketo instance, restart your system after cleaning the cache and then try creating a new draft.
May have something to do with the email template. Vaguely remember encountering a frustrating error a long time ago with the email draft where the template was the issue. Sorry couldn't be more helpful
Please file a Support ticket with the screenshot of that error message. It allows us to look into the log files to find more information about the error that is being thrown. You probably wont be able to discern much detail otherwise. Thanks!