Back to your original question though, It looks as though maybe your SQL Server became unreachable while trying to perform that action. Or it could be trying to access a table that is unavailable. You previously mentioned that you just upgraded from v7.4 to v7.5.9. A lot has changed between those builds. Go through the "Master Settings" section of your forum control panel, and confirm that all the settings are complete and are correct. Then attempt your backup again. In addition, as Gizmo mentioned above, if you have a huge database, the processing of that script could be timing-out, making the entire process prematurely quit. Gizmo mentioned to instead backup using the correct tools for such large databases.
Here's a link of a similar issue error message, which may not be related to your issue at all. But I'm posting it here for others who might find it useful.
https://www.ubbcentral.com/forums/ubbthreads.php/topics/167866/forum-upgrade-error
