There's a likelyhood that some trace got damaged from the leaking capacitors where those caps were.

If you haven't done so already, I would do general continuity testing. You want to make sure electricity is getting to where it's supposed to go (e.g. no broken traces or damaged components), and that there are no short circuits.

Thank you Tiido and Shponglefan; this is the advice I was looking for, though was hoping to get something more specific.

Regarding the MIDI test from the Service Manual there should be IC's 1, 2, 9, 10, 11 involved. From what I can see, only IC11 is close enough on the mainboard to one of the caps (C12)
I tested continuity for IC11 and all traces seem to work fine. Attached is a photo of that section; maybe you can see more?

Can you suggest another area I should be looking for?

Pin 57 and 59 have traces that look suspicious. 57 has some soldermask removed but looks like it can be fine but 59 has that dark spot right at the chip pad and it likely has nothing but corrosion in it. You'll want to see if there's any other such things seen around the other capacitors that got replaced.

Hello Tiido, thanks again!

I agree, that some traces look rather suspicious and would require some treatment, but testing continuity was showing that these are working OK. I also tried to review the areas around the other caps and tested continuity, but also here no faulty traces detected so far.

So this is really hitting the nail of my post: I cannot think of anything else then doing what you recommended (general continuity testing), but since I am not too familiar with electronics, and also having limited capabilities, I was hoping that somebody could help me isolating things.
Meaning I could try testing ALL areas of the mainboard (very time consuming if you do not know what exactly to look for except for broken/corroded traces) while knowing that only some parts of that board maybe relevant to the MIDI circuit.

As said, according to the service manual there are only a few ICs which seem to be part of that MIDI circuit, and only one of them (IC11) is close enough to one of the leaking caps I replaced while testing some suspicious traces still have continuity and others have been tested also. So far I wasn't able to detect where the problem sits...

So, it would be great, if somebody could again direct me to the right spot of the board, where I could continue my investigation?

What I am looking for is something like "if you connect a midi keyboard via midi cable to one of the inputs, and you press a key on your midi keyboard, there should be a signal showing on pin XX on connector YY, and this should also show further on pin mm on IC nn" or similar.

OR as the service manual suggests "when connecting midi out/through of the mainboard with one of the inputs, what should I see in regards of signals/voltages and where? As said, the test fails, but I should be able to trace down what is expected by the test???