SSH server rejecting private key - how to proceed

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Love Postico. I'm a paid user. I'm having issues connecting via SSH and I'm not sure how to debug and correct. What should I look into fixing?

Postgres is running on AWS RDS in a private subnet. I have a bastion server AWS EC2 instance that is publicly available to connect to via SSH private key.

I have successfully used Mac's SSH with my private key and connected to the EC2 server. Then, on that EC2 server, I installed psql and using PSQL on that server, I can connect to the Postgres database on AWS in the private subnet. So:
- SSH allows me to EC2 server
- AWS RDS rules allow EC2 server to connect to Postgres
- Postgres is allowing the connection

It seems that outside Postico, things seem to work as I expect them.

## What did you do?
Add New Favorite
Picked "Connect via SSH" from "Options" dropdown
Configure Host, User, Password, and Database for the AWS RDS Postgres server (validated above)
Note that Host is a big long AWS resource string like myservername.awsidentifiernumber.us-west-2.rds.amazonaws.com

Used the IP address of the EC2 server for SSH Host. (validated above)
Left User, Password blank.
Used 22 (the default) for the Port

Used "Choose" button to choose the private key in ~/.ssh/keyname.pem. I validated that they key worked with command line SSH.

Then, clicked "Connect".

## What did you expect to happen?

I expected to connect _through_ the EC2 server and into the Postgres server running on AWS RDS.

## What actually happened?

Got some sentences flashing in quick succession as the connection is being established, but ending with "The SSH server rejected the private key." I could not see the steps prior to I can't give any more details. If I knew if/where Postico might log stuff, I could look there.

## What software versions are you using?
EC2 runs Amazon Linux 2
RDS running Postgres 14.1

Postico version:
Version 1.5.20 (8940)

macOS version:
Mac OS Mojave 10.14.6

PostgreSQL version:
RDS running 14.1

Start with Postico's Add New Favorite flow using “Connect via SSH,” the selected ~/.ssh/keyname.pem private key, and the SSH connection messages described in the report. Compare the failing flow with the user's successful command-line SSH connection and determine what diagnostic information is needed. Done means identifying the cause of the key rejection and documenting or implementing a verified resolution.

Written by the indexing model from the issue text.