Hi Pieter,
I get the same problem, and since I just now installed it, I think I would have used the updated version? It says version 2.3.1, and installing works, but it refuses to activate with the error message below.
In case it’s important, this is on two sites that have not yet been updated to WP 5, they’re at WP 4.9.9.
It gives me 1 notice, 2 warnings, and 1 fatal error.
(replaced our own server path with […])
Notice: Use of undefined constant __DIR__ – assumed ‘__DIR__’ in /[…]/wp-content/plugins/classic-editor-addon/classic-editor-addon.php on line 29
Warning: include_once(__DIR__/vendor/autoload.php) [function.include-once]: failed to open stream: No such file or directory in /[…]/wp-content/plugins/classic-editor-addon/classic-editor-addon.php on line 29
Warning: include_once() [function.include]: Failed opening ‘__DIR__/vendor/autoload.php’ for inclusion (include_path=’.:/opt/plesk/php/5.2/share/pear’) in /[…]/wp-content/plugins/classic-editor-addon/classic-editor-addon.php on line 29
Fatal error: Class ‘WP_Dependency_Installer’ not found in /[…]/wp-content/plugins/classic-editor-addon/classic-editor-addon.php on line 30
Hope this helps, but let me know if I can provide more info!
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Els