It would seem that someone is changing privacy settings. The two tags I looked at, boy and ashe (top two in your screenshot) didn't show any results when I checked the other day (see my first post). Checking them now shows 7 media for the boy tag (all of which I can view) and 3 for ashe (none of which I can view, though I can see the thumbnails).
I'd suggest that you check the user log to see if the owners of the two albums have recently been changing album permissions, which would explain why the tags gave 404 errors to Google. I'd also suggest you rebuild the caches in the ACP and then see if the ashe tag brings up results for guests (ie when you're logged out). It it still does and the album still isn't available to guests (which it currently isn't) then you may want to report this as a bug.
ashe | Sphynxlair