In fact we are already doing this (clarification: we are using the url returned by the forum site.json so if https we should always be correctly using https even if entered via http), I just found and fix an issue at least concerning iOS keeps logging users out when HTTPS site added via HTTP link not sure yet about the OP error. I will make sure it works and make a release and see if it fixes this error too.
@chrismalone can you give me your forum URL? in private if necessary. And also the exact string you are entering when you see the error please.