For reference
Using Brave Browser for Mac, BB v1.40.107, I tested the opening of a Guest window, with no problem results.
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Opened a New Window, and then used the hamburger menu (upper right corner of window area) to select ‘Open Guest Window’
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Opened a New Private Window, and then used the hamburger menu (upper right corner of window area) to select ‘Open Guest Window’
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Quit Brave Browser . . . wait . . . start up Brave Browser; in the startup panel, I selected the Guest mode button
For each test, a new Guest window opened - no problem.
These steps only confirm a single case for Mac OS users.
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Meanwhile, please go to: brave://settings/?search=crash
Enable: Automatically send diagnostic reports
And then test - repeat your steps - in order for crash report data to accumulate.
@Mattches may want you to locate Crash ID numbers and provide them, here.
An example:
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