thank you for your helpful comments.
Unfortunately I am not equipped for such insulation resistance tests – maybe I have to ask the electricians who from time to time test the equipment in our offices for faults with grounding.
I see some small bits of copper corrosion in the open spots of the platen, so I will first try to enlarge the scratches a tiny little bit for better accessibility and clean the part in these spots. This is not heavy corrosion, only a tiny little bit (in the visible area).
In the extreme I could rip of the plastic film and clean the copper surfaces. But then I would have to apply a new plastic sheet and the current sheet seems to be considerably thinner than the self-adhesive plastic films I know (e.g. DC-Fix, a German brand).
I tested the capacitors only for capacity which is as specified, but not the leak current. I think my meter also shows the ESR value.
Replacing the capacitors would be no problem – maybe something to try before tinkering with the platen.