btrfs fi show displays all btrfs file system but also a lot of errors like:
parent transid verify failed on 109973766144 wanted 1823 found 1821
The filesystem can still be unmounted and remounted.
Surfing the web I found a lot of answers recommending to clear btrfs' internal log by using btrfs-zero-log. I thought btrfsck could help but eventually I discovered the official recommendation which is to first just start a btrfs scrub before taking other actions!
So just run btrfs scrub /path/to/mountpoint if you too can still mount your filesystem. And maybe it works like in my case.