Got fatal error during xfer (Unable to read 4 bytes) in BackupPC

This error is usually caused by incorrect authentication configuration, and is usually seen in rsync and tar transfers when ssh keys are not set up correctly. The ssh server (BackupPC client) returns a password prompt that BackupPC doesn't know how to deal with.

The way to correct this is to make sure the (server) backuppc user has ssh keys generated and copy the (server) backuppc user's public key to the (client) backuppc user's ~/.ssh/authorized_users file on the client. Please read the BackupPC Client Configuration documentation. Instructions for how to set up ssh keys can be found my post.