Comment 40 for bug 1848797

Due to the end of life (EOL) of Bionic 18.04 (https://ubuntu.com/blog/ubuntu-18-04-eol-for-devices), the issue with "Error: symbol 'grub_file_filters' not found." has become relevant in recent days.

In my experience, when upgrading an Azure VM from Bionic 18.04 to 20.04, the upgrade may fail due to the boot mode (UEFI mode or legacy BIOS mode), which is a crucial factor.

If your previous VM was in Generation 1 and you upgrade to 20.04 LTS, it will switch to UEFI boot mode. As documented in Microsoft's troubleshooting guide (https://learn.microsoft.com/en-us/troubleshoot/azure/virtual-machines/chroot-environment-linux), it's important to create a Rescue VM in the same generation and perform rescue operations to switch back to Legacy BIOS mode (see the 3rd point in the guide).

It's worth noting that Generation 1 Ubuntu 20.04 comes with legacy mode, whereas Generation 2 Ubuntu 20.04 comes with UEFI mode. Therefore, if your previous machine was in Generation 1, you should create a Rescue Linux VM in Generation 1 and perform the operation as described by @Ronzo in comment #31.

In summary, to resolve the "Error: symbol 'grub_file_filters' not found." issue when upgrading from Bionic 18.04 to 20.04 on an Azure VM, it's important to consider the boot mode, create a Rescue VM in the same generation, and switch back to Legacy BIOS mode if necessary.