SSH FAIL_FILE_CORRUPTED

Hello,
I used to have following configuration: SSH keys in RDM have private/public key as usual and assigned username. On OS I would have that username and public key in authorized_keys. That worked.
However, yesterday decided I'll go single key for my homelab, to simplify everything, and while normal shell, like iTerm is working, I simply can't get RDM to connect, I always get the above message.
I would expect it to work like this:
I save the key as usual in RDM, but do not assign a username.
In the session, I configure username/password, then I only set username, and under SSH key I select the key I have saved in RDM.
However, I get the above message.
I have found no other way to get it to work with having one SSH-Key saved in RDM, and using different usernames on servers (since, well, SSHKey is not bound to a username, except that it must be under authorized_keys of that username).
So, maybe it would work if I would create multiple ssh-keys in RDM each with it's own username... but... that is a little weird, isn't it?

EDIT: also tried the alternative method I used before, username in the key-entry, and still getting that error... what is wrong?