Backup problem

Hi there, I finally decided to upgrade to 1.5, but before I do I have to backup the database. I have the backup/restore plugin so it should be easy, but I do get the following error message when I login to my wp-admin panel and click the backup database button.:
Warning: fopen(/home/virtual/site136/fst/var/www/html/backup/30032005wordpress.sql): failed to open stream: Permission denied in /home/virtual/site136/fst/var/www/html/wp-admin/backupRestore.php on line 220

Warning: fwrite(): supplied argument is not a valid stream resource in /home/virtual/site136/fst/var/www/html/wp-admin/backupRestore.php on line 221

Still it says:
Backup Successful.

Which by the way is not right. Nothing has been backuped.

So, anyone know (please say yes!) what’s the problem and how to solve it?

Thanks, Boneless
http://www.boneless.nl