Your comments

Great story – I'm glad you like the app! :)
I had a look at it and I know that iSSH does somehow show a transparent keyboard, but unfortunately I found no documented way to do that. If someone knows how to achieve a transparent keyboard in iOS: please tell me.
Great to hear that you could resolve the issue with your FTP conneciton.

Regarding FTPS: Did you disable "Verify SSL certificate" yet? You need to disable this setting if you're using self-signed certificates.
Never heard of that error message before :)

Please enable the "Debug Log" setting in the connection's settings and try to download the file again.

Textastic should create a log file in the local "logs" directory. Please send this file to support@textasticapp.com so I can have a look at it.
This is displayed when you go to the root directory in the Files section and choose "Help". It is shown at the bottom of the help screen.
This will be possible in Textastic 2.2 which is currently in review at Apple: tap-and-hold a file and select "Preview" – this also works for other file types like PDF or Word files.
Thanks, it's great to hear you like the app! :)
Thanks, I'll consider it. Chmod is also a command that's missing from the ftp client.

Currently, you have to download the files and upload them again to move them to a different folder on the server.
Adding custom syntax definitions will be added in Textastic 3.0.