PuTTY key format too new

 Key too new :: Support Forum :: WinSCP

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.