Upgrading error

Hello. Have a problem with upgrading WP version.
System asks for FTP data and after that just informs me – upgrade incomplete (i don’t know how it is exactly in english cause i have translated version.)
I turned on logging and now i can see two extra lines:

Notice: Undefined variable: strings in /data/sites/gomaailm.ee/htdocs/wp-admin/includes/class-wp-upgrader.php on line 97

line 97 in that file represents:
return new WP_Error('fs_no_folder', sprintf($strings['fs_no_folder'], $dir));

Can someone explain, what could be wrong?

When I try to delete some installed plugins from plugin list, i get on header:

Notice: Undefined variable: title in /data/sites/gomaailm.ee/htdocs/wp-admin/admin-header.php on line 13

Notice: Undefined variable: pagenow in /data/sites/gomaailm.ee/htdocs/wp-admin/admin-header.php on line 49

Notice: Undefined variable: is_iphone in /data/sites/gomaailm.ee/htdocs/wp-admin/admin-header.php on line 66