I’m not sure whether it will help anyone but what worked for me was (Note that I do have a multisite):
1) Install the classic editor plugin
2) Network deactivate the CMS Masters Mega Menu, Contact Form Builder and Content Composer plugins.
3) Deactivate the plugins on each seperate website in the network (in my case Dutch and English versions)
4) I get a popup where W3 total cache tells me to empty the static cache, which I do.
5) Activate the plugin only on the seperate webpages (no network activate) and only activate the CMS content composer (I haven’t tried activating the others).
So in short, deactivate the plugin everywhere, empty the w3 cache, activate only on the websites (no network activate).
When I then go to the editor I have a button to activate the content composer which then works again.
Don’t ask my why it worked, I just know that it worked for me and hopefully it will help others.
