I can repdoduce the problem here, and it's not
erratic at all, in fact
the behavior is 100% consistent. If gmidimon or another app already has
the MIDI port open, then aseqdump will get -EBUSY (aka "Resource
temporarily unavailable").
If you had tested these apps one at a time then this would be obvious.
I'm absolutely sure that I have succeeded, one time or more, in monitoring
the same midi device with gmidimon and aseqdump simultaneously.
Regards
Philippe