Filezilla FTP keeps getting blocked

The reason is that FileZilla uses too many concurrent sessions to upload files - basically flooding the web server with requests to the point where the server thinks it is being attacked and then it blocks you temporarily so that you don't ruin the server for all other users sharing the server.

If you are using FILEZILLA as your FTP client, do the following:
1. EDIT (command + , -> transfers)
2. SETTINGS
3. TRANSFERS
in that window modify the following parameters, to have all the same value:
Maximum Simultaneous Transfers: 3
Limit for concurrent Downloads: 3
Limit for Concurrent Uploads: 3
or modify the value until you found one that is ok for you. In my case 3 is the optimum with my Internet connection and with my server.

Also do this for your specific FTP site

1) Start FileZilla
2) Click File
3) Click Site Manager
4) Select your FTP Site from the left-hand menu
5) Click the Transfer Settings tab
6) Check the box for “Limit number of simultaneous connections” and select the number of connections you wish to limit FileZilla too. We recommend no more than 3.
That’s it!

Doing this you will stop those blocked errors with your FTP transfers.

http://www.101sitehosting.com/members/knowledgebase/1846/Filezilla-FTP-keeps-getting-blocked---why.html