When updating or pushing a repository that have subrepos which distant access requires a password, there is a prompt asking for a login and password of each one of these subrepos.
The problem is simply that this prompt don't tell which subrepo it is about, or even if it's about a subrepo or not (as it's the same prompt with non-subrepo push in distant repo that required password to be written in).
Reading the log don't helps identifying which repository it is. The "realm" name is not enough as several repos can be in the same "realm".
My suggestion is to simply add the name (or path or whatever) of the repo for which a login/password is asked in the prompt.