Thanks to the hint from a user on Reddit, I finally found the right TRRRS cables for the MIDI port of the Seqtrak 
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So I ordered two of these cables: First, an extension cable, so I can place the bulky Yamaha dongle away from the device. Second, a longer cable with two TRRRS plugs, which I will cut in the middle to solder on “normal” MIDI plugs.
The product names of these cables: DELOCK 85700 + DELOCK 85697
Here is the internal wiring of the Yamaha adapter that I already posted a few months ago:
