Comment 8 for bug 1889556

We had this problem yesterday on only one of two Ubuntu 18.04 instance on AWS. We had to rollback to a previous snapshot to make the instance bootable again.

So we did a quick test with these two AMI:

ubuntu/images/hvm-ssd/ubuntu-bionic-18.04-amd64-server-20200408 - ami-0edd51cc29813e254
ubuntu/images/hvm-ssd/ubuntu-bionic-18.04-amd64-server-20190722.1 - ami-0d0eaed20348a3389

We deployed one fresh instance for each of those AMI and ran the same update process that we usually do (we the apt module from ansible to perform the updates). After the reboot, both instances had the issue (being stuck at "error: symbol `grub_calloc' not found").

Then, we did the same thing, but ran the apt upgrade manually on both instances and selected the default option on every prompt that shows up. After the reboot, both instances were normally accessible, without any sign of the issue.

Hope that this information can be useful!