The remote server returned an error: (425) Can't open data coection

I implemented ftp servers on Azure platform - Window Server 2012 following http://www.intstrings.com/ramivemula/articles/enable-ftp-access-on-windows-azure-vm/ link. In summary, I created ports (2500-2504) and used them for FTP services. In same time, I created dataport (1000-1005) and registered them through azure port as endpoints. The coection works fine, but sometimes it retus "The remote server retued an error: (425) Can't open data coection." error.

Any idea? Thanks you in advance.