(Since I don’t see anyone in this thread actually confirm a fix, why is this thread marked as resolved?)
I see that the last post to this thread is 10 months old, but I am still experiencing what I believe is the same problem on my multisite installation with the Domain Mapping plugin.
https://mymainsite.com/hiddenlogin , then enter credential, it goes to https://mymainsite.com/wp-admin. Works fine.
But
https://anyothersite.com/hiddenlogin , then enter credentials, it goes to https://anyothersite.mymainsite.com/wp-admin (in the url address field) but shows a 404 page. Note that if I have ‘theme compatibility’ enabled with the default settings, I get https://anyothersite.mymainsite.com/not_found in the url field.
Near as I can tell, I’m not getting logged in at all when I try logging into anyothersite. When I log in through mymainsite, I am then able to successfully navigate to https://anyothersite.mymainsite.com/wp-admin.
I found a two month old post that offers a potential patch/fix:
https://www.ads-software.com/support/topic/http-404-with-hide-backend-and-multisite-not_found-potential-fix?replies=1
But I don’t really have the time or inclination to mess with fixing this problem myself, unless there are no alternatives.
Looking through this and other forum postings, this problem seems to be pretty persistent for the last year or so without a successful fix. Would rather see iThemes fix this problem or find a new security plugin. Has anyone found another security plugin that offers similar features that aren’t broken?