I setup a proftpd server that uses TLS/SSL certificate for authentication.

Everything works well when I connect through lftp or Filezilla (with explicit connection). But once I attempt connecting with simple ftp connection from Filezilla, the USER command ends with the 550 response (SSL/TLS required). After that any further connection through lftp or Filezilla (with explicit connection) will hang authenticating. Anyone knows how to workaround this issue? Is there a way to ask Filezilla to automatically use TLS/SSL if required?

I am using Ubuntu server 10.04 with proftpd 1.3.2c. There is no error message in the log files.