martin wrote:

But it's not like the hostname starts with a space, right?
The space in not in the hostname and you have typed it by mistake. And PuTTY just gracefully ignores it. Is that correct?

Yes, but as WinSCP leans heavily on PuTTY (e.g. import config from PuTTY). I would expect the same behavior. If PuTTY would not accept it either this would be fine. But different behaviors make the error hard to understand.

But it's not like the hostname starts with a space, right?
The space in not in the hostname and you have typed it by mistake. And PuTTY just gracefully ignores it. Is that correct?

I used the nice feature to import the session definitions from PuTTY.
After an AWS workspace upgrade I had redefine my PuTTY sessions. Also WinSCP had to be "redone" and I imported all the sessions from PuTTY. However, I was facing the issue that the PuTTY sessions worked fine but but the WinSCP ones did not (complaining Host does not exist).

When checking WinSCP.ini I noticed that the HostName was actually something like

After correcting WinSCP.ini removing %20 at the start of the HostName the connections was working fine.

I could live with this in case PuTTY would also not support this, but it does and the way things currently are means you can't always rely on sessions imported from PuTTY will work.

I have tested this with WinSCP-6.5.5-Portable.