Scene7 limits uploads to ten simultaneous uploads from any one source IP address. This error is probably because other users in your company are also uploading at the same time. There is another possibility that your FTP client is configured to open simultaneous connections. If you look in the options menu, you see this setting.
For example: In CuteFTP Pro, it is in Tools > Global Options > Connections. The Per Site Max Connections/Transfers option is set to four by default.