Could you set the compression level for image archive to Maximum (Tools -> Options -> Default Backup Options -> Compression level) and try to back up again?

If the issue persists, please try the workarounds described in the articles below:

[url]http://support.microsoft.com/kb/177078/en-us[/url]
[url]http://support.microsoft.com/kb/892100/en-us[/url]

If the issue still persists, please collect the information requested by Alexey in the previous post and submit a request for technical support. Attach all the collected files and information to your request along with the step-by-step description of the actions taken before the problem appears and the link to this thread. We will investigate the problem and try to provide you with a solution.