I had almost the same thing happen in my 20H killed everyone and cracked my tank along with melted my Aqua Clear.... I have just gone with the Petco brand heaters since then and " knock on wood " hasn't happen yet and my water is always typically perfect temp.
This is a well documented issue from almost a year ago on other forums. They had a bad batch and the lot numbers are out there. I don't have them handy, but google it and you'll find them. I have several in use and checked to make sure I didn't have those lot numbers. My tanks and heaters are fine.