ssh-add results in invalid format

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest version
• Search the existing issues.

Steps to reproduce

ssh-add

Expected behavior

should add the created keys to ssh-agent

Actual behavior

results in a invalid format:

Could not add identity "D:\Users\Johannes/.ssh/id_rsa": invalid format
Could not add identity "D:\Users\Johannes/.ssh/id_ed25519": invalid format

Error details

>Get-Service ssh-agent


Status   Name               DisplayName
------   ----               -----------
Running  ssh-agent          OpenSSH Authentication Agent

Furthermore:

ssh-add -l

error fetching identities: invalid format

### Environment data

```PowerShell
Name                           Value
----                           -----
PSVersion                      5.1.19041.4648
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.19041.4648
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Version

OpenSSH_for_Windows_8.1p1, LibreSSL 3.0.2

Visuals

No response

The issue names no source files or tests. Start by reproducing ssh-add and ssh-add -l with the Windows ssh-agent service running in the reported OpenSSH 8.1p1 and PowerShell 5.1 environment; done means explaining and correcting the invalid-format failure so the keys can be added and listed.