Spent some other time on this desperate case. The signal FSRD on J208.2 is sent by the DC board and goes briefly up, but soon after the safety circuit kicks in and cuts the current. I have thoroughly inspected almost everything and everything looks fine. Now, according to the service manual, the current is cut in case of shorted triac Q101 (Page 2-26 “Protection against short-circuited Q101“). I tested it and to me it seems OK, too. But maybe it’s indeed NOK. So, next week I will try to replace both the triac and the solid state relay, as I could easily source both.