@tizz: Once again, thanks very much for jumping in to help others.
@claredotcom: It sounds like if you activate the plugin, it will cause the error, so I think you have two choice.
1) If you want to use autoupdate, try this: go to your plugin page, click the box to select NextGEN Gallery in the list, then select deactivate from the bulk drop down and apply. By doing this, if you now go and rename the NextGEN plugin folder back to normal, the plugin should stay deactivated and you won’t have the error. Then you can update the plugin from your plugin list, and only activate it again once updated.
2) You can also update manually. Just download the plugin from the NextGEN main page on WP.org, unzip the file on your computer, and upload the files to the nextgen folder. Then rename it again. If you’re not too familiar with FTP, the process is the same as you’ll find on our rollback page here: https://www.nextgen-gallery.com/how-to-rollback-a-version/
*If I were you, even if going this route, I’d probably still go to the plugins page, select NextGEN, select deactivate from the bulk drop down and apply.
Give those a try and let us know.