I appear to be having issues when visiting child sites from my Network Profile.
When I go to the child site (seen so far on SO and SU) I appear to not be logged in:
After clicking "log in", it takes me to the login page and quickly auto-redirects me:
When I'm redirected back to the site, I'm again not logged in.
Not sure what additional details are required - I'm on OSX Yosemite, using Firefox 36.0.1. I don't have any addons that would relate to redirects, cookie or session changes or generally rewrite a page.
When I check out my cookies on SU, I appear to have one for acct and another for usr both for ".superuser.com". Unsure if these are private, so I won't post them. Interestingly, there also appears to be a usr cookie for "superuser.com", however this one has no value, whilst the other two do.