I'm trying an upgrade from Drupal 7, for a multisite install.
Tried with two sites a couple of years ago; worked at first.
This time, with one site, just no success.
- which turned out to be primarily because of an issue uploading the Backdrop files.
I saw there was an interruption during transfer; but somehow thought ok. When I later re-uploaded, the website "appeared".
[I have done several Drupal upgrades without this issue]
Also had secondary issue, but solved [as found I'd had same couple of years ago; forgot] by removing non alphaneumeric character in the mysql database password.
Condensed version of the issue I found, just in case anyone happens to have the same in future:
Using a new folder, simply named backdrop - with the Backdrop files, including Sites folder.
It was like this on Home page, and any other page other than update.php it seems, including admin:
Undefined class constant 'LayoutContext::USAGE_TYPE_ALL'
Error: Undefined class constant 'LayoutContext::USAGE_TYPE_ALL' in Layout->getContexts()
Warning: file_put_contents(temporary://fileDpzTF4): failed to open stream: "BackdropTemporaryStreamWrapper::stream_open" call failed in file_unmanaged_save_data()