Well, it used to work with domain names / host names for the past 10 years but it looks like it does not anymore.. or there is a bug somewhere. All the latest versions are always buggy.
WinSCP can of course work with hostnames too. Without verbose logs both from WinSCP and the "SSH", we cannot provide more information.
Not sure what was wrong but I fixed it myself like this. I had it email@example.com and it was not accepted. The domain name is OK, it is not expired, it exists and it is up and running fine. I changed it to user@ip-address like this firstname.lastname@example.org and it all was accepted and works fine now. It looks like Winscp doesn't like domain names in letters, it only "wants" IP addresses. Hope it will help somebody else with the same issues.
by other devices I mean logging in via SSH on another computer, another tablet, another telephone using keys.
Please post logs both from WinSCP and the "terminal".
Regarding the "other devices" – That's unclear. How do you connect from them? Using WinSCP? Or the "terminal"? Or otherwise?
I really need somebody's help / advise. I have a WinSCP@user.com user that has been SFTP connecting using keys and there is also an option to login using its password. If the keys are not loaded then this password may be used. Now something has switched somewhere and neither keys nor typing the password in are accepted. Although I can log in via SSH terminal OK (not WinSCP), using both either keys or password. I can also login from other devices using either keys or password. It's just WinSCP that won't allow this user to login. I tried changing the password for WinCSP@user.com because I am root but it does not help. WinSCP still won't allow that user to login even with a new password. What is wrong? How do I fix that? Is that a fingerprint's issue? I have never seen this before and I am totally lost. How do I restore this user's access via WinSCP? I would be really grateful for any help / assistance / advice. Many thanks in advance!