Cannot back up host system - permission denied

[Syndicoon]

I have two servers - A and B.
Dockerized version of backuppc was set up on server A and started right up - no issues.
I configured both servers (A and B) the same way with a user (backuppc) and the ssh key.
Backuppc is able to do a full and incremental backup on server B without any issues.
Server A fails with the following (Xfer LOG):
NOTE: I use the ip addresses for the usernames - but this works for server B.

XferLOG file /data/backuppc/pc/x.x.x.x/XferLOG.0.z created 2022-03-23 05:30:54
Backup prep: type = full, case = 1, inPlace = 1, doDuplicate = 0, newBkupNum = 0, newBkupIdx = 0, lastBkupNum = , lastBkupIdx = (FillCycle = 0, noFillCnt = )
Running: /usr/local/bin/rsync_bpc --bpc-top-dir /data/backuppc --bpc-host-name x.x.x.x --bpc-share-name /mnt --bpc-bkup-num 0 --bpc-bkup-comp 3 --bpc-bkup-prevnum -1 --bpc-bkup-prevcomp -1 --bpc-bkup-inode0 2 --bpc-log-level 1 --bpc-attrib-new -e /usr/bin/ssh\ -l\ root --rsync-path=sudo\ /usr/bin/rsync --super --recursive --protect-args --numeric-ids --perms --owner --group -D --times --links --hard-links --delete --delete-excluded --one-file-system --partial --log-format=log:\ %o\ %i\ %B\ %8U,%8G\ %9l\ %f%L --stats --checksum --timeout=72000 --include=/ x.x.x.x:/mnt/ /
full backup started for directory /mnt
Xfer PIDs are now 506
This is the rsync child about to exec /usr/local/bin/rsync_bpc
Permission denied, please try again.
Permission denied, please try again.
root@x.x.x.x: Permission denied (publickey,password).
rsync_bpc: connection unexpectedly closed (0 bytes received so far) [Receiver]
Done: 0 errors, 0 filesExist, 0 sizeExist, 0 sizeExistComp, 0 filesTotal, 0 sizeTotal, 0 filesNew, 0 sizeNew, 0 sizeNewComp, 2 inode
rsync error: unexplained error (code 255) at io.c(226) [Receiver=3.1.3.0]
rsync_bpc exited with fatal status 255 (65280) (rsync error: unexplained error (code 255) at io.c(226) [Receiver=3.1.3.0])
Xfer PIDs are now
Got fatal error during xfer (No files dumped for share /mnt)
Backup aborted (No files dumped for share /mnt)

What I have checked:
Directory permissions on both servers to compare settings. They are the same.
backuppc directory and .ssh directory permissions
checked the .ssh_config to make sure backupp wasn't 'locked out'
Deleted, and recreated the backuppc use on server A manually in case permissions were corrupted.

I configured this with the assumption that docker will act like a separate server that interacts with the host (server A) like it does with an actual remote server (Server B). Or is it unable to back up the host at all? Or is there a special configuration needed that I cannot find anywhere?
Server A is my main server and has files on it that I would like to backup. Although I have a backup already, I would like to automate the process for a second backup to the backup if you will.
I am stumped.