Ah, thank you. They were probably generated and exported with puttygen indeed.
Is there a way to load and re-export them to adhere to the correct format? (provided that this will keep the fingerprint used in ~/.ssh/authorized_keys unchanged - otherwise I will regenerate them from openssl and redeploy, something I would like to avoid because I have not automated that part)
You can use sshpk-conv for that task, available on Debian-like distros at the node-sshpk package, like this: sshpk-conv /path/to/your/private/key -t ssh -p.
Thanks, sshpk-conv worked to convert an id_rsa file originally generated on Ubuntu 20.04, successfully imported into Bitwarden.
For anyone else finding this - a little extra clarity on what’s actually happening, here’s the relevant portion from the sshpk-conv man page linked above:
pem, pkcs1
The standard PEM format used by older OpenSSH and most TLS libraries such as
OpenSSL. The classic id_rsa file is usually in this format. It is an ASN.1
encoded structure, base64-encoded and placed between PEM headers.
ssh
The SSH public key text format (the format of an id_rsa.pub file). A single
line, containing 3 space separated parts: the key type, key body and optional
key comment.