1. Successfully upgraded to 3.9.25. System was backed up just before this upgrade. But only partially - see note below.
2. Then started to upgrade extensions.
3. Joomla stopped displaying message:
Warning: require_once(/homepages/13/d210002123/htdocs/aa6e.net/nw/plugins/system/backuponupdate/tmpl/default.php): failed to open stream: No such file or directory in /homepages/13/d210002123/htdocs/aa6e.net/nw/plugins/system/backuponupdate/backuponupdate.php on line 331
Further attempts to login to the admin backend (https://aa6e.net/nw/administrator) immediately repeats the message above.
Front-end site still works correctly. (https://aa6e.net)
(I have since learned that my backup was not taken correctly. It only copied a subset of my site. So I cannot restore it fully.)
Thanks for any advice. I hope that there is a simple action that will re-enable the admin site!