FTP protocols and technologies require a network of hardware and technology to establish the communication between your computer and hosting server. Transfer errors occur when a file is corrupted during the transfer at some point and cannot be read by the server. 

In such cases, you will get an error report with the list of files with errors, and you should be prompted to attempt to transfer again. Under some configuration specification, you must wait for the connection to re-establish itself, which could take a few minutes. 

If an error occurs with the same file several times, or that a file is blocked by the server, check that the source file isn't corrupted or if you must change the configuration of your hosting server to accept the type of files you are trying to transfer. 

Also, check that you haven't exceeded the maximum capacity for your hosting server, in which case any new file transfer would be blocked.