This sounds a bit weird, but I tried connecting to a FTP server with the exact same credentials on my iPad, iPhone X, and iPhone XS. Only the iPhone XS doesn’t work, shown in the attachments.
The error page shows up instantly. I also don’t see any activity in the FTP server log when it happens. I’ve also tried reinstalling the app and turning off iCloud sync. Not working.
Tried the Raeddle’s Documents app. It cannot connect to the FTP server as well. However both Infuse and Documents can connect to the SMB server on the same machine.
Turns out it was a fail2ban setting in the NAS that banned the access from the iPhone XS… This is awkward.