I also think it's coming from the remote side.

Loging in with

su -s /bin/bash - backuppc 

with the '-' as you suggested does not stop the backup from working on the 
command line.

I also installed backuppc on another server and tried to backup another client 
(so, 2 different machines involved here), and again the same error message.

------------------------
Fatal error (bad version): /sbin/nologin: invalid option -- 'c'

Usage:
 nologin [options]

Politely refuse a login.

Options:
 -h, --help     display this help and exit
 -V, --version  output version information and exit

For more details see nologin(8).
------------------------

Something is being executed on the remote side with option '-c', maybe 'su'. 

I get the same message when I try to run

su backuppc -c ls

from the backuppc server as root

------------------------
nologin: invalid option -- 'c'

Usage:
 nologin [options]

Politely refuse a login.

Options:
 -h, --help     display this help and exit
 -V, --version  output version information and exit

For more details see nologin(8).
------------------------

Fuji

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