A few days I try to upgrade a site with Drupal 7 to Backdrop 1.9.0, I have done it many times and always occurs the same problem - the site is upgraded, but Administration menu is corrupted. Dislocated items, absent items.
/admin/content is not available at all - it is impossible to see content list.
I follow literally the instructions here:
(It seems that one step there is missing - before "6. Navigate to this site in a browser, and visit /update.php to proceed with the update." it was necessary for me change in settings file $settings['update_free_access'] = TRUE;)
I remembered that some time ago - beginning of last year - I have done a successful upgrade from Drupal 7 to the earlier versions of Backdrop.
So I went back and did the upgrade procedure again, but to Backdrop 1.2.3.
The upgrade has become successful and now there are no problems with Administration menu.
After that I made an update to Backdrop 1.9.0. Now the upgraded site looks and works fine.
So I think there's a problem with the upgrade from Drupal 7 to Backdrop 1.9.0.
Sorry, I didn't make screenshots.