I contact Red Gate support earlier today about this. They pointed me to a different (later) version of SQL Prompt on their FTP site. After I uninstalled the version I had and installed the new one the problem went away. I'd recommend you contact Red Gate support. They were very responsive when I contacted them by phone.
The fix that Red-Gate provided earlier is no longer working. The same error is occurring again. I'm on 18.104.22.168. I think I updated SQL Prompt a few days ago and that must be when the problem came back. Time for another Red-Gate ticket.