Recurrent backup error

I installed the plugin for the first time yesterday, but no of backups I started succeded.
The reported errors are very similar to these reported in the log below.
I tried to follow some of workarounds I found online, checked my disk quota and looks like I have plenty of space.

Here is the info from help & support page, below you will find the last log from my backup.

Online Backup for WordPress Plugin Version 3.0.4
WordPress Version: 3.6.1 (standalone)
PHP Version: 5.3.3-7+squeeze14
Server Software: Apache
MySQL Server Version: 5.1.49-3
MySQL Client Version: 1.0; MySQL Packet Size: 1 MiB
Memory: 256M; Post: 32M; Upload: 32M; Timeout: 120
Memory Changable: Yes; Admin Cache Size: 152.09 KiB
Status: 1 row, 18.1 KiB packet size, 149.99 KiB cache freed, 40.94 MiB memory used
Capabilities: hashcopy gzdeflate php5hash
End of Information

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6th ott 2013 11.11.16 AM Error The activity failed: A filesystem operation failed.
6th ott 2013 11.11.16 AM Error A filesystem operation failed while processing ‘Filesystem/home/mhd-01/www.xxxx.it/htdocs/wp-content/uploads/2012/09/VT5-300×229.jpg’ (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: 1441(/home/mhd-01/www.xxxx.it/htdocs/wp-content/plugins/wponlinebackup/include/files.php)
An error happened at: disk.php(290)
fopen(/home/mhd-01/www.xxxx.it/htdocs/wp-content/plugins/wponlinebackup/tmp/gzipbuffer.83e438d7ce50c59db218c2b62a1c77a63e3dc3fc.5.php): failed to open stream: Disk quota exceeded
6th ott 2013 11.06.19 AM Information File system scan completed.
6th ott 2013 11.06.14 AM Information Folder /htdocs/wp-content/backups was excluded: Local backups directory
6th ott 2013 11.03.52 AM Information Folder /htdocs/wp-content/plugins/wponlinebackup/tmp was excluded: Temporary backup directory
6th ott 2013 10.59.13 AM Information Database backup completed.
6th ott 2013 10.59.09 AM Information Initialisation completed.
6th ott 2013 10.59.09 AM Information Ignoring comments that are marked as spam.
6th ott 2013 10.59.09 AM Information Ignoring comments in the trash.
6th ott 2013 10.59.08 AM Information Backup starting…

http://wordpress.org/plugins/wponlinebackup/