How to break stop Unity 2020.1's Hold on message?

While game playing, I clicked ctrl+z few times,

and now pop up msg says

Hold on (busy for many seconds)…

Application.something…

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It seems to start doing that after having Unity running for awhile.
Closing and reopening the engine works for me.

But then, unsaved scene works all gone?

Ah, do you mean that it gets stuck in this state endlessly? Because that’s different from what I was referring to.