if switch matrix is working fine then try this one:

the signal from the 2 opto-isolators on the powerdriver is going to J113 pin 1,3,5.
From there to the CPU J211 pin 1,3,5
J211-1 is connected to J206-1 (column drive 1)
J211-3 is connected to J209-2 (sw. row 2)
J211-5 is connected to J209-1 (sw. row 1)

so, you should have continuity from
PDB U7-5 to CPU J209-2 (think this is left flipper) and
PDB U8-5 to CPU J209-1

if these points are connected and switch matrix is ok, then the problem is powerdriverboard.
Can be R210 / R211 or U7 / U8.

hope you can use this,
Sascha

checking boards, checking schematics, buzzing traces.....No, I have no explanation for that

That's a coincidence. Just yesterday I checked this topic of 7 months ago because there's a small error in one of the pictures I made and wanted to know if in this topic J211/J113 was correct described.
Luckily I used the multi-meter instead of the manual/schematics that time so it was correct.
The Manual mixed up pin 3 and 5.
J211-J113.jpg

Great it help you too, Schudel5.
For future use a more visual description of the connections.
pre-Fliptronic.jpg

Then you need this one , it's with the latest corrections.
wpc89-cpu-parts-id-okt14.pdf

4N25 maybe ?

https://nl.rs-online.com/web/p/optocouplers/0597289

or pick one up at the DPM when you're in the neighbourhood

Wait with U7

you can make a short between the "c" and "e" on U7 and U8 on the powerdriverboard during switch test . That is pin 4 and 5 . That should trigger left and right flipper.
if not, somewhere traces are missing.

if it does trigger L + R flipper, then the next step is to check voltage on the large resistors R211 + R212. There should be +72Vdc on the left side of the resistor.
if not, replace resistor. If there is +72 Vdc present, then it is time to replace U7 / U8.