This issue, or a similar one, seems to have reappeared in Buster. My system is a HP ProBook 650. I installed Stretch a while ago, and then upgraded to Buster as per instructions on Debian website.
After a while and a few security upgrades the system was unable to boot, spewing out error: symbol `grub_calloc“ not found. Entering rescue mode... grub rescue> I am not very proficient in grub rescue commands, and felt quite lost. After a bit of tinkering I found this workaround: The laptop has a "Boot from EFI file" option in "Boot devices" menu in BIOS. Navigating to and selecting [efi partition]/EFI/debian/grubx64.efi booted Grub just fine. After a while I got fed up doing this on every cold boot. Then, out of curiosity, I tried selecting [efi partition]/EFI/boot/BOOTX64.EFI from the "Boot devices" menu in BIOS. The same Grub error resulted, i.e: error: symbol `grub_calloc“ not found. Entering rescue mode... grub rescue> Then I booted the system again, backed up /boot/efi/EFI/boot/BOOTX64.EFI and replaced it with /boot/efi/EFI/debian/grubx64.efi and rebooted again. Hey Presto! The boot problem vanished. If I remove BOOTX64.EFI from /boot/efi/EFI/boot altogether, I get a "Boot device not found" error from BIOS on boot. I hope this helps in correcting this situation. Regards ;)Frode -- Da sa Gud: "Det bli lys!" Og det ble lys. 1. Mosebok 1.3 And God said, "Let there be light," and there was light. Genesis 1:3, NIV