It look like the UUID of the root partition is wrong or hash changed for some reason. So you need to fix/update to the proper UUID.
This can be done by booting into a Ubuntu Live-CD and open a konsole and type in the following commands:
$ sudo blkid
/dev/sdb1: UUID="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" UUID_SUB="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" TYPE="btrfs" PARTUUID="xxxxxxxx-xx"
/dev/sda1: UUID="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" UUID_SUB="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" TYPE="btrfs" PARTUUID="xxxxxxxx-xx"
The output of the command might be a bit different according to your disk partion tabe. In most cases the
/dev/sda1 is the root filesystem. So you need to mount the partition in the running live system and change UUID in the /etc/fstab file
$ sudo mount /dev/sda1 /mnt
$ editor /mnt/etc/fstab
# /etc/fstab: static file system information.
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
# <file system> <mount point> <type> <options> <dump> <pass>
# / was on /dev/sda1 during installation
UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx / btrfs defaults 0 0
The filesytem type and might be differnt in your case. If
/dev/sda1 does not contain the
/etc/fstab, then you have the wrong partition.
umount /mnt and try the other partitions.