quotas: failure on removing a file via SFTP/SSH

[ST <smntov@gmail.com>]

Hello,

I'm trying to use quotas for a simple chrooted sftp setup, limiting
space for each user's subvolume (now for testing to 1M).

I tried to hit the limit by uploading files and once it comes to the
limit I face following problem: if I try to free space by removing a
file via Linux sftp client (or Filezilla) - I get error:
"Couldn't delete file: Failure"

Sometimes, but not always, if I repeat it for 3-5 times it does removes
the file at the end.
If I login as root and try to remove the file via SSH I get the error:
"rm: cannot remove 'example.txt': Disk quota exceeded"

What is the problem? And how can I solve it?

Thank you in advance!