This test report is indicating a fault reported from the fliptronics blue and grey wire (F4). This could be a fault with the left flipper opto board, so check that.
http://techniek.flipperwinkel.nl/wpc/index3.htm#opto

You might also have bad connectors or cracked header pins on the Fliptronics board, check those first
-then-
(this is also from that text link, btw)
On WPC fliptronics to WPC-S board, chips U4 and U6 (LM339) on fliptronics board can fail. On WPC-95, these LM339 chips are on the CPU board at locations U25 and/or U26. This will make the flipper opto boards seem like they are not work. Swap the two flipper boards to test this. If the problem doesn't change, suspect the LM339 chip(s). These LM339 chips can also become "leaky". This will make flippers seem very weak. A bad LM339 can also give the indication that the EOS switch is bad.

You are wise to ask and proceed slowly.

Asfar as a visual inspection is concerned, you'll need some good magnification. An example of what to look for is in the pic below. Also check for loose cables or bad crimps on the connectors. This could be as simple as oxidized connectors. If nothing else is found the LM339s are a common trouble part. If the LM339s are socketed they are easily replaced with an IC pulling tool. If they are soldered, you are going to need the required tools and experience to get them desoldered and extracted.

kalte-lotstelle_128006 (resized).jpg

Probably a good point. I assumed these may have been checked. I'm just not sure what kind of error report you get with that fuse blown. I could pull that fuse later on my FT to see what happens. Fuses usually blow for a reason, so that's something to consider.