I've just rebooted an AWS instance running buster and it has failed to restart, with the message "symbol 'grub_calloc' not found" shown on the console screenshot.
Just to wrap this up: Thanks to Noah for your advice. I have written up what I had to do to repair this at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=966575#214 Lessons for debian-cloud: - I bet there are still affected systems that haven't been discovered by their owners! I encourage another grub update. I worry particularly about Lightsail instances, where I think recovery would be much more difficult than EC2. - Symlinks in /dev when disks change from xvd* to nvme* would help. - I spent a lot of time confused about how to re-attach the root volume, because the AMI's root mount point was "xvda" without the /dev prefix. Is this due to something in the Debian image? Can it be changed? - Fundamentally I think that grub being split into core and modules in this way is an accident waiting to happen; could the AWS images find some other safer way to boot? Regards, Phil.