By "main caps" do you mean the line supply filter caps,right after the
rectifier? The raw DC supply for the switcher? If it's a doubler,you
should have around 300VDC,if a single-ended rectifier,about 170VDC.
Perhaps you don't have enough input DC. Or maybe you're seeing some AC
along with the DC.

Many SMPS use a cap-rectifier doubler for 120 VAC operation,and a switch
changes it to single-ended for 220VAC operation.

The PSU is 250Vac: the two capacitors are 680uF/200V (they seem to be
series-connected) and they show, with no load, 215V across each capacitor.
This value is suspect: I would expect 170V, as you stated, so the problem
could lie in the primary side of the PSU.

The 680uF/200V is mandatory ? Can I try to substitute them with, for
example, 1000uF (or more)/400V ? I have them handy ...