I have been working with my new Backdrop site (a freshly upgraded Drupal 7 site) and have been having an issue editing the layouts "Manage Blocks" page
/admin/structure/layouts/manage/default
When I click any "add block" item, I only get a short whirlygig, and no pop-up. The "configure" links work.
I checked Permissions and all seems okay.
Help appreciated!
Dan
Oh, one more thing. You have Ubercart installed. So, UC in BAckdrop famously has three broken views when you upgrade from D7.
You can try taking out any views files that start with
views.view.uc_...
until you find the culprits.