There are other discussions on this but there does not appear to be a concrete solution. I am hoping that someone can help me with a solution and then I can also document this better.
When I go to add an image to an email with kcfinder Browse Server option I get a popup with a 404.
The URL in the popup https://crm.mysite.org/libraries/civicrm/packages/kcfinder/browse.php?cms=civicrm&type=images&CKEditor=crmUiId_1&CKEditorFuncNum=0&langCode=en appears to be correct.
This has me thinking that the issue is the Drupal htaccess rules.
The files for kcfinder were not in the libraries/civicrm/packages/kcfinder folder by default but I added the files from another Drupal 8 install.
I am not sure there is other information I can provide that would be of use to solve this.
Thanks, Steve
composer civicrm:publish
didn't run properly during install. You can try running that on its own.