After the upgrade from Jessie to Stretch, GRUB started throwing the following error:
error: no symbol table. ... Press any key to continue...
If I pressed any key or didn’t press anything at all, system would boot normally after a few seconds. Still, the error message bothered me, so I fixed it by using the solution proposed here.
In my case the Linux drive is sda, so I ran the following commands:
grub-install /dev/sda update-grub
and all was good again.