Hi Steven,
UpdraftPlus backups are standard zip files. The error message below indicates that the currently installed “Jetpack” plugin is faulty. This lives in wp-content/plugins/jetpack. So, to replace that with the version from your backup, do this:
1) Find your “plugins” UpdraftPlus backup. (i.e. the zip file with “plugins” in its name). Unzip it, and you will have a directory called “plugins”. Inside this, there should be a directory “jetpack”.
2) Use FTP to log in to your WordPress site. Navigate to the “wp-content/plugins” directory. Delete the directory “jetpack”
3) Upload the “jetpack” directory from step 1, so that it becomes the new wp-content/plugins/jetpack
Best wishes,
David