I had the same problem, and my problem was resolved after upgrading to version 5.19.
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
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".