I have the same problem and tried the fix that you suggested. However in the site settings area there is nowhere to change that setting.
The system instead of appending a path to site admin is replacing the whole URL with a path beginning with the site admin location. This is the same as the problem that the OP had.
You are correct, it's the [SiteSettings]AdditionalLoginFormActionURL setting in site.ini you need to change. It's probably defined in your front end siteaccess(es).
