Your comments

Thanks a lot for the feedback and the test. It gets even more interesting. I have tried with my synology nas server, which is local in my LAN. SSH, irb, works fine. Well, a very old ruby version 2.4.2 is installed, but I cannot imagine the version makes the trick.

When I go back to all the other servers, it does not work, as mentioned, this „blinking triangle“ appears, and the session is blocked. I have to mention that for this I always connect via IPSEC vpn  (the iPad embedded vpn service) to the remote location.

As mentioned before, with Secure CRT on iPad the same thing works flawlessly. I work on those servers everyday, usually from Mac. Now I wanted to setup the same thing with Textastic.

Any idea what could cause the issue or how to tshoot further?

did you check with Textastic on the iPad? I have just checked with Secure CRT on the iPad. Secure CRT is a very well known ssh terminal app. With that it works flawlessly. It‘s also not an issue on the ubuntu server running ruby, I have tried with several different ones. It works on Secure CRT on iPAd but also on any other PC, Mac, Linux connecting to the servers. Only Textastic shows me this „triangle“ and hangs. 

I am only talking about textastic on the iPad, I have never tried it with Textastic on other plattforms.