Same thing for me. All my daily backups stopped working on May 26 (but my weekly backups have continued just fine). I just today discovered the 1969 “next run” date on the dailys, so I just resaved the schedule tab and we’ll see what happens. This behavior was identical on three different web sites running on different hosts. All were running WP v3.5.1 (until today, now v3.5.2) using BackWPup v3.0.12. I’m anxious to see what happens after tonight’s daily run.