It took me about 45 minutes, and 2 phone calls to figure out that Windows Firewall apparently suffered some corruption, blocking me from FTP to this server. Not all FTP servers (for some reason that’s beyond me). All clients failed. First I thought it was the server. Then I thought it was a networking issue (net admin said FTP was not blocked). In the end, it was Windows Firewall. Resetting it using the Default Settings button brought me back to life (for anyone experiencing this problem that may be helpful to know).
I normally SFTP these days, but sometimes FTP for some reason (likely habit more than anything).