OpenSSH key marked as invalid when trying to establish a SSH connection with duplicity

Question

I'm trying to automate a backup process using duplicity initiated by a bash script.

The data is to be tranferred via SFTP. To automate the authentication, I have set up keys using ssh-keygen -t rsa and transferred the public key to the server. Establishing a SSH connection the regular way works without problems.

I have this line starting duplicity to transfer the data: env PASSPHRASE=$(cat pass.txt) duplicity $target_dir sftp://$USER@$bak_server/$store_location. The constants being the paths and addresses.

And it works fine on any GNU/Linux but doing this on Mac leads me to these frustrating debug messages:

Backend error detail: Traceback (innermost last): File "/usr/local/bin/duplicity", line 1560, in <module> with_tempdir(main) File "/usr/local/bin/duplicity", line 1546, in with_tempdir fn() File "/usr/local/bin/duplicity", line 1385, in main action = commandline.ProcessCommandLine(sys.argv[1:]) File "/usr/local/Cellar/duplicity/0.7.18.2/libexec/lib/python2.7/site-packages/duplicity/commandline.py", line 1135, in ProcessCommandLine backup, local_pathname = set_backend(args[0], args[1]) File "/usr/local/Cellar/duplicity/0.7.18.2/libexec/lib/python2.7/site-packages/duplicity/commandline.py", line 1010, in set_backend globals.backend = backend.get_backend(bend) File "/usr/local/Cellar/duplicity/0.7.18.2/libexec/lib/python2.7/site-packages/duplicity/backend.py", line 223, in get_backend obj = get_backend_object(url_string) File "/usr/local/Cellar/duplicity/0.7.18.2/libexec/lib/python2.7/site-packages/duplicity/backend.py", line 209, in get_backend_object return factory(pu) File "/usr/local/Cellar/duplicity/0.7.18.2/libexec/lib/python2.7/site-packages/duplicity/backends/ssh_paramiko_backend.py", line 235, in __init__ self.config['port'], e)) BackendException: ssh connection to user@IPv4:22 failed: not a valid RSA private key file

The duplicity version on the server and on my Mac are the same. Trying this with my server as client to another server also works fine.

Is there a known cause/workaround/solution for this? :/