For some reason, like most MCDzienny servers I've seen today has had this heartbeat failure. I also noticed that the Minecraft Classic Server amount went down to like 70 when it usually much higher than that.
It's confirmed. I fixed it in the new version, please update. It turned out that Mojang disabled/removed http version of the heartbeat page, and there's only https one now. It's interesting that they decided to mess with Classic.