If you have root access to the endpoint, the easy way to solve such problems is to run sshd in debug mode, by issuing a command like
On the endpoint:
sudo `which sshd` -d -p 2222
on the endpoint
On the FlexDeploy Server:
if trying to connect using user/password
ssh -p 2222 user@host -v
or if trying to connect using an ssh key with or without a passphrase
ssh -p 2222 user@host -v -i <private key file here>
Between the two outputs, you will probably be able to figure out what the problem is. Otherwise, send both logs in a support ticket.