been on this ALL day. no repeatable success unless there are NO MIDI devices at all.
Monitoring the console indicated that the USB MIDI driver for my Yamaha ES8 wasn't 64-bit, so I updated that.
There are no differences in console messages during startup of MainStage with the following exception:
during v2.2 startup it immediately posts two messages
16:26:08 [0x0-0x80080].com.apple.mainstage: MTEngine
16:26:08 [0x0-0x80080].com.apple.mainstage: MTEngine>
I don't know what this is, but v2.1.3 doesn't post these messages on the console.
additionally,
OS X seems to recognize that the process has hung as it posts this message on the console and creates a logfile
16:37:22 spindump:
Saved hang report for MainStage version 2.2. (419.23) to /Library/Logs/DiagnosticReports/MainStage followed by a timestamp, then the name of this machine ".hang"
To say that I'm flabbergasted by this is an understatement.
I thought to give reinstallation a thought, so I deleted v2.2 of the app, also removed it from the Launchpad (I never use this, it's just that the App Store installs here).
Then it got even weirder... unlike the iOS app store, the Mac app store app insisted that MainStage was already installed on this machine, and offered no way of reinstalling...
I restored v2.2 then from my nightly clone of the startup disk.
Should there be someone who can read the "spindump" I'd be happy to post... but I'm truly not a happy camper at the moment.