Okidoki.
Unfortunately I had to reboot before I saw your message, but it seems to have succeeded in installing a majority of the updates it found on 12th of May.
I have attached a screenshot of recent updates - unfortunately it's in Danish, but "Fuldført" = "Succeeded" and "Mislykkedes" = "Failed". And "Anbefalet" = "Recommended" and "Vigtig" = "Important".
So, only the latest Security Essentials-update seems to be acting up. I will do a reboot and see if it corrects itself.