Security errors when redirecting to other sites

Hi There - I have a network wide issue. I'm not sure it relates to Pro Sites, but the worst thing relates to the checkout. What is happening is this - my main site / network admin are secured with https. Many of the other sites are not. But, redirects from the main admin to other sites try https and give a security error. The worst offender here is after someone pays for a site, the link that redirects them back to their site does this - so right after checkout they get a security error. Also, from the network admin if you go to My Sites and try to go to the dashboard of any other site it happens. In fact, most places that link to other sites from inside the network / main site's admin do this. What can I do?