Look at the authlog message "Failed password for ppetra"". It's entirely possible it's not an ssh incompatibility issue. It could be that ppetra's password has expired.
So if he can do a SSH login from Putty but not from mobaxterm/cygwin with an error complaining about password, I'd be looking at the mobaxterm/cygwin configuration(s) for setup errors, e.g. protocol settings.
can others (you perhaps) login using those tools?, if so then there's most likely something wrong specifically with user X's configuration as intimated by @hicksd8