That was some interesting input, yes obviously these particular metal-paper capacitors from Rifa are responsible for quite some damage in electronics - Swedish quality from
Radioindustrins Fabrikaktiebolag...
First of all, to me the sequence of the events is quite clear and it started from the capacitor, because what is not clear from the pictures I posted is that the board was originally mounted vertically with the thyristor directly
above the caps.
Given that probably nobody in 1976 expected this electronics to be still in daily use in 2015 the caps have survived quite well. They are encased in epoxy, and even without cracking some humidity might have diffused into the cap over the years, compromising the self-healing functionality.
When it comes to the voltage rating - these caps are actually not installed
over the mains voltage, i.e. not between phase and neutral. But they will be replaced by modern 630V rated caps