Admin panel white page due to fatal error in admin-header.php

Hiya,

I upgraded my PHP (from 5.x to 7.x), then tried to update my WP install to 5.2.2. (currently 5.1.1.) but something went wrong. The site still works, and I get the admin login window and successfully log in (meaning that on the public site I see the admin ribbon on top of the page) but the admin dashboard and the whole backend is all white/blank.

I turned on WP_DEBUG, and it gave me the following:

Fatal error: Uncaught Error: Call to undefined function wp_is_recovery_mode() in [path]\wwwroot\wp-admin\admin-header.php:55 Stack trace: #0 [path]\wwwroot\wp-admin\index.php(111): include() #1 {main} thrown in [path]\wwwroot\wp-admin\admin-header.php on line 55

The line 55 in the said admin-header.php reads as:

if ( wp_is_recovery_mode() ) {
/* translators: %s: Admin screen title. */
$admin_title = sprintf( __( ‘Recovery Mode — %s’ ), $admin_title );
}

When I comment that out temporarily, I then get the admin backend working fine, except that on top of every page there’s these error messages:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘wp_recovery_mode_nag’ not found or invalid function name in [path]\wwwroot\wp-includes\class-wp-hook.php on line 286
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘paused_plugins_notice’ not found or invalid function name in [path]\wwwroot\wp-includes\class-wp-hook.php on line 286
Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘paused_themes_notice’ not found or invalid function name in [path]\wwwroot\wp-includes\class-wp-hook.php on line 286

Any ideas what might be wrong and what I need to resolve this (or can I just leave that bit commented out and ignore the error message)?

A related supplementary question: can I try again to update to 5.2.2., is it likely to resolve this or make this worse?

Thanks in advance,
robaa