That is approx. 87% and with BTRFS 80% is already critical. You are in the critical range where there will be problems and where you need to make adjustments. Just like on ext4 at 90%.
Btrfs offers powerful features like fast snapshots (Timeshift), but these also consume metadata space. If no more metadata can be written - meaning no new metadata chunk of 256 MB can be allocated - then no additional data can be stored, even if free space appears to be available. For example, the system may report 5.49 GB of free space, but this may be reserved exclusively for 1 GB data chunks. At this critical point, a rebalance is required: data must be redistributed across existing data chunks and then free up completely empty chunks afterward, which can then be reused for new metadata chunks.
I hope that makes sense to you. Some sort of defragmentation of data chunks, not the data itself, to get similar meaning.
Check how much space is allocated:
sudo btrfs filesystem usage /
If not enough space is unallocated, you would need to (re)balance:
#Start it:
sudo btrfs balance start -musage=90 -dusage=90 --bg /
# Watch the progress:
sudo watch -n1 'btrfs balance status /'
Anyhow @andreas85 explained it also:
A final word: If you are using BTRFS and snapshots, keep an eye on the filesystem to make sure it is about 20% free.