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.