March this year I posted about this phenomenon.
The real problem is that now, after installing SP1, also the dynamic send connections are left open (they are closed after the session timeout). As it is a dynamic port a new connection is opened with every file send. Due to the limited amount of connections getting into trouble is inevitable. So now the whole concept of a dynamic FTP port becomes rather useless when only limited connections are available.
Recently Microsoft came with the hotfix for this issue. for this issue.