The most recent cPanel update threw FTP offline on all servers. The update they completed last night included a fix for this bug. All should be working normally again now. Our apologies for the trouble.
I need to upload files from different sources here, so I can only do say 20 files at a time. Today I got just two lots, and it stopped while doing the third.
However, I can still use that one at a time feature in my account. Pity I have a lot of files to upload, as it would be very slow doing them one at a time.