virtualbox.org

My host operating system is Windows 8. I recently upgraded VirtualBox from version 4.2.12 r84980 to version 4.3.0. Following the upgrade, existing VirtualBox sessions no longer open. (I get a 'failed to open session' error). I reverted to version 4.2.12 and am again able to load my existing VirtualBox sessions. Is this a known issue?

What does "session" mean to you? If you mean saved state then yes, it's a known issue that they are not always compatible across versions. You work around by right clicking the VM and choosing "Discard saved state". Personally I never use them at all.