@philm I can confirm that modifying /etc/default/grub and running update-grub did not fix the issue, like in @TrueRandom’s case. I’ve been reading related threads today (both old and recent) and had tried those fixes separately and together.
No dual boot, I’m on testing, the error first appeared after the last round of updates (which I installed yesterday), the system boots successfully after pressing a key or just waiting 10 seconds. Interestingly, that error also appears when the system’s shutting down.
@TrueRandom, I suggest going through the main thread shared by philm where this is being discussed: Can't boot LUKS encrypted install after 2023-03-31 stable update
As of now, the solution reported as working most times seems to be the one described here or here, but running sudo grub-install does have some risks (also reported in the thread), so worth thinking about twice.