RuntimeException (400) after GRAV update

I get this error after I've updated GRAV and plug-ins:
Template "partials/langswitcher-bts.html.twig" is not defined in "partials/navigation.html.twig" at line 31.
Previous exceptions
Template "partials/langswitcher-bts.html.twig" is not defined in "partials/navigation.html.twig" at line 31. (0)
What is the problem?
How can I by means of admin page restore the site from backup?

PS: During update there were also some CRITICAL log messages noticed:
1. "Use of undefined constant GRAV_WEBROOT - assumed 'GRAV_WEBROOT' (this will throw an Error in a future version of PHP)"
2. "Too few arguments to function Grav\Plugin\Admin\Router::__construct(), 1 passed in /home/xxxxx/public_html/user/plugins/admin/admin.php on line 286 and exactly 2 expected"

Start with the missing templates partials/langswitcher-bts.html.twig and partials/navigation.html.twig, then review user/plugins/admin/admin.php around line 286 alongside the listed update log messages. Done means identifying the update or plug-in incompatibility and confirming a workable admin-page recovery path.

Written by the indexing model from the issue text.