If you’re using Arq 7 to connect to a macOS Ventura SFTP server and you’re getting the error “Username/PublicKey combination invalid”, it may be due to Ventura’s upgrade to OpenSSH 9.1. The release notes for OpenSSH 9.1 state:
This release disables RSA signatures using the SHA-1 hash algorithm by default. This change has been made as the SHA-1 hash algorithm is cryptographically broken, and it is possible to create chosen-prefix hash collisions for <USD$50K 
To work around this, you could edit
/etc/ssh/sshd_config on the Ventura server and add this line to allow use of your existing key:
Or, you could create a new key using a different algorithm. Arq 7 supports ed25519 keys, so you could
ssh-keygen -t ed25519
to create a new public/private key pair. Add the public key to your account’s
~/.ssh/authorized_keys file on the Ventura server, and configure a storage location in Arq using the private key.