I once had a problem with an AKAI MX73 master keyboard. The MIDI out was badly designed (to use one TTL chip less), so I got problems with longer cables or cables with less quality. (Any other MIDI device worked fine with these cables.)
I found out, that the 1N4148 diode in parallel to the optoisolator input degrades signal quality. A Schottky-diode (like a SB160, SB190, SB1100) instead of the 1N4148 works fine (because it switches faster), even with lower quality MIDI input signals. So I replaced the 1N4148 diode on my homemade MIDI interfaces with a Schotky-diode.
So it might be the diode, which disturbs the signal reception.
So try to replace the diode at the optoisolator first, either same type 1N4148 or Schottky-diode (a bit higher voltage, 50V should be enough, 1 Ampere is more than enough). If this does not help, try to replace the optoisolator too. If this also doesn't help, the problem is probably not in the MIDI input circuit.
Good luck!