I have run into this error message when trying to restore a course and the backup file is larger than the file size allowed by the php.ini on the server that hosts the Moodle site.
If that is the issue, you have a couple of options.
If you can edit the php.ini file, change the "post_max_size" and "upload_max_filesize" to a larger number, such as 20MB or 50MB.
If you can't edit the php.ini file on the server, try creating new backups of the course you need to restore, such as one backup with just the forums, and one backup with just the assignments, or a backup for each half of the course, or a backup without the userdata, and then do multiple restores from the multiple backup files.
If you have the option to not include any large image, presentation, or video files in the backup, that may also help. You will then have to add them to the restored course separately.
Hope this helps someone!