@yash: Please give us (way more) details than "not opening up". PuTTY event logs from both scenarios, for example. WinSCP application log (use /applog=C:\path\app.log command-line switch).

I've updated PuTTY to the latest version.

Also it is working fine using normal PuTTY.

@gclaborn: Looks like you have an old version of PuTTY.

Such a strange occurrence. I've used this key to login to servers running CentOS, Alma, Rocky and Debian, but for some reason on Ubuntu WinSCP can use the key while launching PuTTY from WinSCP fails. Launching PuTTY manually with a different session name and same key fails.

While WinSCP is benefiting in my case; I wonder if the same difference caused your situation. Something apparently works differently than PuTTY's session start does, though they share the same settings panel. Very odd.

Thanks for your report.
This issue is tracked here:
Issue 1952 – Support rsa-sha2-256 and rsa-sha2-512 SSH public key algorithms
(as a donor, you have an access to nightly builds, where this is resolved already)

Winscp 5.19.4, connecting to OpenBSD 7.0 stable.
transfer protocol SFTP
Utgåva Windows 10 Pro
Version 21H1
Installerad ‎2021-‎02-‎06
OS-version 19043.1288
Gränssnitt Windows Feature Experience Pack 120.2212.3920.

I have a pubkey that I have used for a long time.
After upgrading OpenBSD to 7.0 stable the key is rejected by the server for
WinSCP but not for PuTTY.
When connecting from PuTTY I get this log message in the sever /var/log/authlog:

When connecting from WinSCP I get this message in the server /var/log/authlog:

It is the same key. Since this is a RSA/SHA256 key it should work also for WinSCP.
(After adding PubkeyAcceptedAlgorithms=+ssh-rsa in the server sshd_config it works also for WinSCP.)
See also:
https://marc.info/?l=openssh-unix-announce&m=161880182302909&w=2
https://marc.info/?l=openbsd-tech&m=163028217802671&w=2