Thanks Kleinstein,
The original mounting is shown in the attached picture. There is a small plastic support under the diode leads (like the marked one in the picture). If a screw comes loose, or the connection gets bad, the support melts away, and the connection gets even worse.
The diodes them selve seem to be fine. I think every one of the melted ones, are from a loose connection. I was thinking about soldering them on the topplate instead. They have really many panels, where the diodes (or the whole plastic case) melts away.
A newer type with the diodes in springterminals seems to last much longer between fails.