I’ve tried fresh installations (twice), flushing all the caches, creating new wp-config and wp-login files, removing the plugins and non-default themes, and moving location in case it was a network/filewall issue.
Yes, the redirection happens after I’ve submitted the form. The username and password appear to be fine, because WordPress DOES still alert me if I get them wrong. When I submit the login form, all that apparently happens is that the page refreshes. But I don’t know if this is an issue with WordPress or 1and1.