WordPress Network: Cookie error when using different domains

This is a recurring I have with my website: I can’t get my WordPress Network to work properly when using different, unrelated domains. Both my personal and professional blog (blog.charliemerland.me and blog.caercam.org) from the same WordPress installation, as well as my Themes Demo site (themes.caercam.org). I read a lot about WordPress MU Domain Mapping, spend …