On the new WordPress 5.0 update, I was able to switch between the classic editor & Gutenberg without installing the “Disable Gutenberg” extension. Looking at the release notes for the “Disable Gutenberg” plugin, it states:
“Do not use Disable Gutenberg with other plugins (like the Classic Editor plugin) that also disable or replace Gutenberg. Why? Because it may cause loading of redundant scripts, which may in turn lead to unexpected/untested results.”
You stated you are running the Classic Editor plugin with “Disable Gutenberg”. This is the conflict. Otherwise, clear your cache in the CMS & browser to see if it makes any difference.
Solution:
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Without installing the “Disable Gutenberg” plugin, navigate to the classic editor settings at: /wp-admin/options-writing.php#classic-editor-options
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Under the settings for “Default editor for all users” use the buttons to toggle between ‘Classic Editor’ & ‘Block Editor’ (Gutenberg). Save the settings & reload the page.