Hi @gracemonahan,
I can only see minor differences between the composer.json-files, specifically
"composer/installers": "~2.0"
"roots/sage-installer": "dev-master"
vs.
"composer/installers": "^2.0",
"roots/sage-installer": "^2.0"
and also that you should move the roots/sage-installer into the required-section (vs. required-dev) but since you say that you have copied the complete composer.json and tried with that to no avail I am clueless as to what might be the culprit here…
Was hoping for some fatal errors in the debug.log – the deprecations won’t cause the white screen of death.
Check your composer.lock-file within the Sage theme directory and make sure that you have the illuminate-packages in version 8.x – e.g.
"name": "illuminate/container",
"version": "v8.83.26",
If the versions there are lower the your composer update didn’t go through properly and you should run that again and post the output here.
Sorry, that’s all off the top of my head…