Multiple users for same AWS-EC2 account

The first thing to do in this situation is to use the -v option to ssh, so you can see what types of authentication is tried and what the result is. Does that help enlighten the situation?

In your update to your question, you mention "on another local Ubuntu". Have you copied over the ssh private key to the other machine?

You cannot transfer an EC2 instance (or any other resources) to a different AWS account.

If the instance is EBS boot (recommended), you might try an approach like this:

1. Stop the current instance (ec2-stop-instances)

2. Create an AMI from the instance (ec2-register-image)

3. Give the second AWS account permission to run that AMI (ec2-modify-image-attribute)

4. Run a new instance of the AMI under the second AWS account (ec2-run-instances)

DNS would need to be updated to point to the IP address of the new instance (preferably using an Elastic IP Address). Any other AWS/EC2 resources would also need to be copied/recreated in the second account.

After sufficient testing, you might want to free up the original instance (ec2-terminate-instances).

The second account should create their own snapshots / AMIs of the instance to protect themselves if their instance/EBS volume fails after the AMI owned by you is deleted.

Even better, you should have documented/scripted exactly how your instance was created so that the client can reproduce this at will.