Alright, I think this crash is now solved. I can't give full credit to 12.5.1 as I think I actually had two different crashes that I thought were the same.
Thanks for your help, Leo. You can stop reading here, if you like. I'm going to detail what I did today in case either of my two crashes come back.
I'm running Opus 12.5.1.
Previously, I would get crashes as follows:
1: Every once in a while opening a new Opus window. Would crash a couple times a day.
2: Every single time I press my [Flat] button, regardless of what directory is active when I do it -- local or network and I tried various directories. The button does this: Set FLATVIEW=On,Grouped. 100% reliable crash.
Prior to today, I had assumed both crashes had the same (unknown) cause but were linked to TortoiseSVN. But when I isolated crash (1) to TortoiseSVN, I suspect that I hadn't yet run into crash (2). I believe that I later discovered crash (2) and just assumed it was the same problem as crash (1), just reproducible every time.
Now that I upgraded to 12.5.1 from 12.5.0, I think crash (1) is gone. My earlier comment said it still existed after my 12.5.1 upgrade five hours ago, but I can't reproduce it now anyway. I can only reproduce crash (2) now. Okay, so what's the cause of crash (2)? This gets slightly weird.
I uninstalled TortoiseSVN and TortoisGit today, then rebooted. I could still reproduce crash (2) afterwards. More evidence that crash (2) is not crash (1), as if I needed more evidence.
So maybe Opus 12.5.1 fixed crash (1). I'm gonna say that's probably the case. And maybe crash (1) didn't produce a minidump until 12.5.0. Okay. Crash (1) does seem to be gone. Good riddance.
Now we have crash (2). TortoiseSVN and TortoiseGit are uninstalled at this point. So I checked other shell extensions using the wonderful shexview.exe. All listed shell extensions were enabled at this point. Zero disabled. I now disabled every shell extension except those from Microsoft. Didn't reboot, just tried to reproduce crash (2) and couldn't reproduce it. Rebooted, still couldn't reproduce it. Re-enabled every disabled shell extension 50% at a time (enable half, try to reproduce the crash, repeat on the remaining disabled extensions). Can't reproduce crash (2). Tried a reboot once I had every shell extension re-enabled. Can't reproduce crash (2) again. So it seems that disabling every non-Microsoft shell extension (they were all enabled before I started debugging this today) fixed crash (2). And re-enabling them didn't (or hasn't yet) re-introduced crash (2).
I reinstalled TortoiseSVN and TortoiseGit (I'm lazily omitting version numbers here, though I do know which versions I _un_installed) and then rebooted (though I wasn't prompted to reboot). Still can't reproduce crash (2). That's it. That's my report. Kinda weird. I hope neither crash recurs and I'm very glad to be able to use [Flat] again.
If you read this far, thanks again for all your help, Leo.
P.S. Somewhere in those steps for isolating crash (2), I wanted to know if 12.5.0 would still produce crash (1), so I downgraded from Opus 12.5.1 to 12.5.0. Not certain where in my steps I did this downgrade, but it was early on. With 12.5.0, it crashed on first run after reboot after the 12.5.1 to 12.5.0 install (therefore presumably crash ) and created a minidump. I decided crash (1) was indeed fixed in 12.5.1 based on this and upgraded back to 12.5.1, wherein I have not yet reproduced a crash on run so I assume crash (1) is truly fixed.