Nevertheless, this was the case here.
The unit came to me because the midi out was failing, it was completely dead. Of course I checked the jack and the connections around it, and all seemed fine. I reset the software.
Just for a lark, I removed the tiny small value capacitor that filtered the midi out line to ground, and found that the midi out came right back.
Although the capacitor did not read as shorted, it's capacitance must have drifted and increased significantly, enough to actually be filtering out the midi signal, rather than just noise.
It was a 220 PF cap, shown below, and the unit worked fine when it was removed, and continued to work fine when a new one was installed. The second picture shows the area where the cap was located... I believe labelled C4 on the silkscreen.