boot - ext4 corruption leading to unbootable disk. Partition is mountable, but grub detects unknown filesystem

I'm trying to recover from a system disk corruption issue without having to reinstall the OS, which would be super-painful currently.

I'm using Ubuntu 18.04, the affected partition is ext4 and on a SSD disk.

Summary:

• Root partition gets remounted read-only after some error.


• df -h shows a huge partition (ZB) instead of the 800GB it should be.


• Boot with a live USB.


• Repair partition with fsck -v. Fixed some errors (orphaned inode and size is xxx should be 0).


• Mount the partition, looks fine. Can navigate, edit files, everything seems to be there and looking good.


• Reboot, the partition is now unbootable. I get the grub console, I can't grub> ls the partition; error is "error: unknown filesystem.". I get the same error from the live USB with "grub-mount /dev/sdb2 /mnt"


• I try to repair using Boot Repair. Unsuccessful, but now I have this log with a report.

Any ideas? To me the inconsistency between successfully mounting the partition with mount and unsuccessfully with grub-mount seems really strange. I guess if I could fix this I would be able to boot normally again.

Thank you a lot.