Transmission can't access mount: Read-only file system

I have a BTRFS external drive mounted at /mnt/media. I can create files/directories just fine with the shell (with both my personal user and transmission user). When I try to download something to the mount, Transmission throws Error: Read-only file system (/mnt/media/sometorrent).

$ su transmission
$ ls -l /mnt
drwxrwxrwx 1 root media    0 Jan 11 08:47 media

$ echo 'test' > /mnt/media/file
$ cat /mnt/media/file
test

badblocks and btrfs check didn't show any errors with the drive. I could use it with Transmission until recently.