Quickly creating an issue for this; proper issue summary will follow.
https://www.drupal.org/docs/8/upgrade/upgrade-using-web-browser#pre-upgr...
If the database credentials to your source database are correct, the upgrade review page will appear as illustrated in the screenshot below. This page shows a summary of the upgrade status for all installed modules on the old site.
Latest iterations use this help text instead:
The new site is missing modules corresponding to the old site's modules. Unless they are installed prior to the upgrade, configuration and/or content needed by them will not be available on your new site. Read the checklist to help decide what to do.
GitHub Issue #:
3577
Recent comments
We can no longer add contrib projects in the Tugboat sandboxes that we use for core PR's? Can this be fixed or is there a reason for this? We can add contrib projects to demo...
Apr 25th Weekly Meetings
The Mail System and MimeMail modules are now installed. I'll let you know if they solve the problem. Edit: Using Mail System with MimeMail I was able to send plain text emails. There is...
HTML Email treated as plain text
If you haven't already I recommend installing Mail System and MimeMail. The latter will help format emails as HTML and first helps with configuring which module will handle the formatting...
HTML Email treated as plain text