Sql Anywhere 11: Restoring incremental backup failure
Summarizing the information linked to in the post above....

The command you have used is for a live backup but what you want is an incremental backup.  The approach you want to use (per this blog) uses somewhat different switches.  You want to use the -n -t -x switches, and the -o to specify a log file.  So the command ends up something like:

dbbackup -n -t -x -c "eng=ServerName.DbName;uid=dba;pwd=sql;links=tcpip(host=ServerName)" -o "c:\backup\backup_log.txt"