BTRFS cannot write to the partition when 40-50% of it is in use

When I use RDFC (available at http://www.bertel.de/software/rdfc/index-en.html) to create files until there's 40-50% free space left on a BTRFS partition, it gives errors about not being able to write to the disk even though there's free space remaining. The debug log is attached.

How to reproduce:

1. Make a 4 GB VM
2. Install ReactOS into it on a BTRFS formatted partition
3. Download RDFC and extract it to the root of the partition
4. Open Command Prompt
5. Type CD \ and press Enter
6. Type rdfc dummy 500 MB and press Enter
7. Repeat step #6 with varying filenames until the errors mentioned in the log start to show