The FTP Server
The FTP server runs a program that monitors the log files for attacks. Whenever it thinks somebody is trying to break in, it reconfigures itself to block that person. Unfortunately, it also has to restart itself to do this — and that kicks everybody off. This shouldn't happen too often, and you can resume file transfers where you left off.
421 Service not available
This usually means that the server has blocked you. Getting a wrong username or password four times in a row will make the server block you for two hours. Either contact Felix (he can un-block you) or try again later.