I can reproduce this with Wavelab 9.1 64 on Windows 10 2004 with latest updates. Apparently it happens in 2004 and 20H2, but I can confirm that it doesn’t happen in Win 10 1909. Enabling Crash Reporting in Wavelab yields nothing. (no report, Wavelab just disappears)
But I wonder if Steinberg would create a patch for a previous version, like Wavelab 9.1, if it used to work ok, but now the Microsoft updates have broken it, which seems to be the case here? Is there a precedent for such a patch to a previous Wavelab version?
I also wouldn’t be surprised if Windows updates fixed this on it’s own at some point, although I wouldn’t rely on that hope for a fix.
Another workaround is to use the FILE TAB (not File Menu) for everything: opening montages and audio files, “import” audio files to montage, and all save-as operations.