@Marcel Is the upgrade to php 7 the only change? Does the file /www/htdocs/[my_user_account]/lists3-ckeditor/ckeditor.js exist and is readable?
But your version of phplist includes an old version of the plugin, which now loads ckeditor from its content delivery network.
Can you either upgrade to the current release of phplist, which is 3.2.6, or upgrade the plugin itself, on the Manage Plugins page, see https://resources.phplist.com/plugin/ckeditor
It took me a while but I was able to solve the problem by
a) upgrading the plugin and
b) changing the settings for “URL of ckeditor.js” and “Path to CKeditor custom configuration file” in the CKEditor settings from relative paths without the respective filenames to absolute paths with filenames.
I mention it just in case someone else runs into the same problem.
What I don’t understand though: In order for phplist to work with php 7 I had to add an open_basedir directive set to “/www/htdocs/[my_user_account]/lists3/” to the .htaccess file in the phpList installation path, as recommended in Phplist and PHP7. In order to be able to upload attachments I also had to add the temporary directory “/tmp” to the directive since it’s outside of the phpList directory. Given that my CKEditor installation is also outside the phpList directory at /www/htdocs/[my_user_account]/lists3-ckeditor/ I thought that I had to add the path to the directive as well but that doesn’t seem to be the case? Isn’t access for phpList restricted to the directories listed in the directive? Sorry if the question is stupid but I don’t really know what I’m doing…