I had the same problem, and it seems to be related to the version of SSH you're running.

If I type

where ssh

I get...

C:\Windows\System32\OpenSSH\ssh.exe
C:\Program Files\Git\usr\bin\ssh.exe

When I run ssh -V in both locations, I get

OpenSSH_7.5p1, without OpenSSL
OpenSSH_7.3p1, OpenSSL 1.0.2k  26 Jan 2017

...respectively

So, when I run ssh from the git/bin directory, it works fine and doesn't complain about the permissions, but running the same command line, using the former installation of SSH, it comes back with this.

Load key "t:\\mykeys\\rich-private.ppk": invalid format
[email protected]: Permission denied (publickey).

ps. the permissions on the file are just full access for myself, and nothing else.

This seems to be related to the version of OpenSSH you're running:

• where ssh returns:

  %WinDir%\System32\OpenSSH\ssh.exe
  %ProgramFiles%\Git\usr\bin\ssh.exe
  

  ssh -V returns:

  # %WinDir%\System32\OpenSSH\ssh.exe
    OpenSSH_7.5p1, without OpenSSL
  
  # %ProgramFiles%\Git\usr\bin\ssh.exe
    OpenSSH_7.3p1, OpenSSL 1.0.2k  26 Jan 2017
  

When running ..\Git\usr\bin\ssh.exe, it works fine and doesn't complain about the permissions, but running ..\OpenSSH\ssh.exe comes back with the following, even though key ACLs are Full Access for myself and nothing else:

load key "t:\mykeys\rich-private.ppk": invalid format
  [email protected]: Permission denied (publickey).