Installing Windows 7 after installing Ubuntu on btrfs: how to restore boot?

I've looked at the answers on this question, but none of them solved my problem:

How can I repair grub? (How to get Ubuntu back after installing Windows?)

My system is partitioned as follows. /dev/sdb5 is Ubuntu's root partition.

I installed Windows 7 after installing Ubuntu. All my Ubuntu partitions (except swap) have the btrfs filesystem.

Installing Windows left me unable to boot in Ubuntu. Running a live USB, the live Ubuntu was not able to find the Ubuntu partitions using os-prober, probably due to this bug.

I'd like to get Ubuntu booting again. Can you help me?