Kludge
Got a failed one on the bench.... replaced U1, R5,
R9 and R11, all of which were bad. On my unit, R5 appeared to be a 10K resistor instead of 1K.
Replaced the mosfet with a STW11NK90Z which was the only 900V one in the junk box, but which looks to be a good equivalent.
The supply does not fire up, and I measure 4.37V on the current sense pin of the U1, which is shutting U1 down as soon as it goes over 1V. What else can be going wrong with the current sense section besides R11 and R5 going wrong?
Is it the MOSFET sub?
U2 tests as being okay (well, at least it's not a dead short, and it tests like a pair of diodes), and U2 is clearly the voltage feedback isolator. But where is U3 and what is it? From your description it's an optoisolator but I cannot find the thing on this very cluttered board.