Working:

service: T-Mobile BIS-B, T-Mobile Wifi (both)
host: proftpd 1.2.10 running on linux
login: usernames + passwords

Not working: anonymous logins on the same server which does accept anon logins. "Could not connect to FTP server" is the dialog error in the client. The server log has nothing more than:

"USER anonymous" 331
"PASS anonymous" 230
"QUIT" 221

On a user based login that is working, the third line item above is a normal "PWD" 257 -' command, so something is failing that I cannot see (bad client command?).

Suggestion: the password default should be 'anonymous@' - many FTP servers require an @ sign in the password field for successful anon logins.