How to exclude directories from being backed up?

I am trying to run the backup, but receive a failure notice after about 65 – 75% of the backup completing.

I am running WordPress 3.9.2 and Online Backup for WordPress version 3.0.4

I suspect it has to do with some large files I have in a sub-directory called ‘ftp’ located at the same hierarchical level as the wp-content and wp-includes sub-directories, but I can’t seem to figure out how to exclude that directory yet backing up what is in the wp-content.

Can you please help?

Here is the error message I get when backing up:

7th Sep 2014 10.43.23 AM Error The activity failed: A filesystem operation failed.
7th Sep 2014 10.43.22 AM Error A filesystem operation failed while processing ‘Filesystem/html/ftp/20140808ARR_Previews.zip’ (Size unknown) for backup.
If the following error message is not clear as to the problem and the issue persists, please contact support providing the complete event log for the activity. Diagnostic information follows:

Failed at: 1045(/home/content/93/9284693/html/wp-content/plugins/wponlinebackup/include/files.php)
33554530
7th Sep 2014 10.07.37 AM Information Folder /html/wp-content/backups was excluded: Local backups directory
7th Sep 2014 10.06.07 AM Information Folder /html/wp-content/plugins/wponlinebackup/tmp was excluded: Temporary backup directory
7th Sep 2014 10.01.51 AM Information Link /html/stats/logs was skipped: Links are not currently supported
7th Sep 2014 10.01.47 AM Information Database backup completed.
7th Sep 2014 10.01.43 AM Information Initialisation completed.
7th Sep 2014 10.01.42 AM Information Ignoring comments that are marked as spam.
7th Sep 2014 10.01.42 AM Information Ignoring comments in the trash.
7th Sep 2014 10.01.42 AM Information Scheduled backup starting…

Here is how I have the General Settings when I attempt a Local backup:

Database excludes:

Exclude comments in the trash.
Exclude comments that are marked as spam.
Filesystem excludes:

Files that are part of the default WordPress installation will always be backed up when the filesystem is included.

Exclude plugins (wp-content/plugins; wp-content/mu-plugins)
Exclude themes (wp-content/themes)
Exclude uploads (wp-content/uploads)

Custom filesystem excludes can be specified here, one per line, relative to the following folder: /home/content/93/9284693/html
For example, to exclude /home/content/93/9284693/html/folder/cache, enter folder/cache into the box below on its own line. Always use forward slashes even if your server runs Windows.

Filesystem/html/ftp/
home/ftp/

I thought the Filesystem/html/ftp entry would do it, but it doesn’t appear to be. Do I need to enter the actual root directory’s name (i.e. the domain name} followed by the ‘/’ followed by ‘ftp’?