Key too new

I use PuTTY and WinSCP to connect to an AWS server. PuTTY runs with no error. But sometimes (for no apparent reason) WinSCP refuses to connect and gives me the following error. With the same settings it sometimes connects and it sometimes doesn't.

Authentication log (see session log for details):
Unable to load key file "C:\Users\navid\OneDrive\Desktop\Keys\private.ppk" (PuTTY key format too new)
Using username "ubuntu".

Authentication failed.

Your key file is generated using PuTTYgen 0.75, which generated keys in a new format, which is supported only by the latest beta version of WinSCP (5.18.5 and newer):
https://winscp.net/tracker/1986

You can use PuTTYgen to convert the key to the older format, which is supported by the current stable version of WinSCP.
In PuTTYgen, go to Key > Parameters for saving key files > PPK file version > 2.

Another way is to load the key to Pageant.

I cannot imagine that you might be getting this error only sometimes (maybe except, if you have the key sometimes loaded to Pageant and sometimes not).

I had the same problem, and my problem was resolved after upgrading to version 5.19.