I've just experienced similar thing as you.
I failed to run "urpmi" with message after updated proftpd.conf:
...retrieving failed: curl: (9) FTP: access denied
But I overlooked the "access denied" and went to check log message, which showed the same strange triplet as you:
FTP session opened.
invalid CommandBufferSize size (0) given, resetting to default buffer size (512)
FTP session closed
I started snort to check the packets and found it's indeed some "Permission denied" condition.
So I suggest that you check again the error message in your ftp client. It's quite probable an access permission problem.