I finally found the guilty for all this situation!!! It's qmail-maildir++.patch, which i patched on qmail to add maildirquota support.
I've compiled a fresh copy of qmail with only this patch applied and bingo! Got the same issue that i described before. I only need to know how am i going to do to add maildirquota to my accounts without this patch... Any idea? Rafael Felix Correa