421 Too many users - please try again later.

Serv-U架设的FTP服务器，用户连接老是失败，提示信息：

FlashFXP v3.3.5 build 1110

WinSock 2.0
[右] 正在连接到 218.66.104.157 -> IP=218.66.104.157 PORT=21
[右] 已连接到 218.66.104.157
[右] 220 Serv-U FTP Server v6.4 for WinSock ready...
[右] USER editor001
[右] 421 Too many users - please try again later.
[右] 连接失败
[右] 正在延迟 120 秒, 之后将尝试第 1 次重新连接
[右] 正在连接到 218.66.104.157 -> IP=218.66.104.157 PORT=21 (尝试 # 1)
[右] 已连接到 218.66.104.157
[右] 220 Serv-U FTP Server v6.4 for WinSock ready...
[右] USER editor001
[右] 421 Too many users - please try again later.
[右] 连接失败
[右] 正在延迟 120 秒, 之后将尝试第 2 次重新连接
[右] 正在连接到 218.66.104.157 -> IP=218.66.104.157 PORT=21 (尝试 # 2)
[右] 已连接到 218.66.104.157
[右] 220 Serv-U FTP Server v6.4 for WinSock ready...
[右] USER editor001
[右] 421 Too many users - please try again later.
[右] 连接失败
[右] 正在延迟 120 秒, 之后将尝试第 3 次重新连接
[右] 正在连接到 218.66.104.157 -> IP=218.66.104.157 PORT=21 (尝试 # 3)
[右] 已连接到 218.66.104.157
[右] 220 Serv-U FTP Server v6.4 for WinSock ready...
[右] USER editor001
[右] 421 Too many users - please try again later.
[右] 连接失败

最终发现原因是Serv-U测试期已经结束，被自动切换成了“个人版”，有诸多限制：