It's Stuart here from the BackupAssist team, thanks for posting on our forum.
Sorry to hear that you're having these issues with your backup and that the suggestions so far haven't worked for you.
I take it that you're performing a Windows Imaging backup - if this is the case, then the error you've been receiving is quite generic.
Usually when these types of VSS errors appear within the Windows imaging engine, there will be a better explanation as to why the errors are being received in the server event logs in Event Viewer (Start > Run > eventvwr).
The best place to start would be the backup logs, which are located in Event Viewer by browsing to Applications and Services Logs > Microsoft > Windows > Backup.
Hopefully the VSS writer which is generating the error will be listed and this will give you a lead as to where you can start looking for a resolution.
If you're still having issues once you've checked the event logs on the system, then you may need to provide us with some diagnostic information from the BackupAssist console so we can investigate further.
To send us diagnostic information please follow these steps:
1) Go to Contact Support (from the bottom-left corner of the console) -> Contact Support
2) Fill out the form and ensure that the Submit system diagnostics checkbox at the bottom is checked.
If you have any issues using our automated support tool please follow these steps to manually send in your diagnostics file:
1. Go to C:\Documents and Settings\All Users\Application Data (server 2003/XP) or C:\ProgramData\ (server 2008/Vista/Win7)
2. Email email@example.com a zipped version of the following folder: BackupAssist vX (Where X is the version that you are running)
If you find this file is too large to e-mail, you can trim down the size of the logs by removing anything older than a week within the following folders under the BackupAssist folder
With the Temp folder, it's the default destination for PST and SQL backups to be stored during the backup process, so if there are any of these types of backups contained you can remove these as well as we don't require them.
If you have any questions or concerns about anything I've said, please don't hesitate to reply to me; I'm more than happy to consider any feedback you have, or answer any questions.